test(harness): make the stub scoping independent of file order (t44)

A subset of test files in an order the suite was not written for failed 3
test_init.py::TestImportStep tests with ModuleNotFoundError: voluptuous, while the
full suite stayed green. Two order-only mechanisms, both now fixed at the source:

1. entity_stublib.install() guarded on ONE sentinel
   (`homeassistant.components.media_player`). A file whose teardown popped
   `voluptuous` but left the sentinel turned the next file's install() into a no-op,
   so importing config_flow failed. The guard now checks every face a file scopes on
   (`_STUB_FACES`, derived from ENTITIES_STUB_NAMES so the two cannot drift) and
   rebuilds the set when anything is missing. install() also registers the
   `homeassistant.helpers.selector` face itself, which the per-file installers had
   been adding separately — without it the derived list would never be complete.

2. Teardowns restored only the names their file had listed, so a module the file
   loaded (`nsh.diagnostics`, `nsh.media_player`) survived into the next file still
   holding stub objects — a stub exception class, the redact helper — that had since
   been replaced; comparisons against the new objects then failed (5 tests in
   test_command_routing on an unlucky order). Every file now ends through
   entity_stublib.snapshot_modules()/restore_modules(), which drops EVERYTHING the
   file created under the harness namespaces (homeassistant/voluptuous/nsh) and puts
   back everything it replaced. Hand-written lists were the bug, so there is none to
   keep in sync.

The t7 no-leak discipline is unchanged: a file's stubs still never reach another
file, and a file's loaded modules still do not survive it. A session-scoped baseline
install was tried first and rejected: files whose own installer guards on "the stubs
are already there" skipped installing their richer faces, which produced exactly the
inconsistent state a baseline is meant to prevent (10 test_standalone_setup failures).

tests/test_harness_scoping.py: the sentinel poisoning, the wholesale-rebuild
invariant, a teardown that removes what the file loaded, the stale-object case
(diagnostics vs the redact helper), and — because order is the thing under test —
two subprocess runs of the reported orderings.

Verified: the reported order (test_command_routing, test_protocol_transport,
test_standalone_setup, test_init) 152 passed / 1 skipped; the minimal pair both ways;
the previously failing 6-test order now 167 passed; every test file in REVERSE
alphabetical order 369 passed / 2 skipped; two scrambled thirds green; full suite
369 / 2 and 370 / 1 with androidtvremote2; ruff clean.

tests/test_harness_scoping.py

@@ -0,0 +1,181 @@
+"""tests/test_harness_scoping.py — the stub harness must not depend on file order (t44).
+
+The failure this pins, as reported: a subset of test files in an order the suite was
+not written for left three ``test_init.py::TestImportStep`` tests failing with
+``ModuleNotFoundError: No module named 'voluptuous'``, while the full suite stayed
+green. Two mechanisms produced it, and both are order-only:
+
+1. ``entity_stublib.install()`` trusted a single sentinel
+   (``homeassistant.components.media_player``). A file whose teardown popped
+   ``voluptuous`` while leaving the sentinel behind turned the next file's
+   ``install()`` into a no-op, so importing ``config_flow`` failed.
+2. A file's teardown restored only the names it had listed. A module it loaded
+   (``nsh.diagnostics``, ``nsh.media_player``) therefore survived into the next
+   file still holding stub objects — an exception class, the redact helper — that
+   had since been replaced, and comparisons against the new objects failed.
+
+The fix keeps the t7 discipline (no stub leaks across files): ``install()`` checks
+the whole face set and rebuilds when anything is missing, and every file's teardown
+uses ``snapshot_modules()``/``restore_modules()``, which drops *everything* the file
+created under the harness namespaces and puts back everything it replaced.
+
+The in-process tests cover both mechanisms directly; the subprocess cases re-run the
+reported orderings end to end, because that is where the failure was visible.
+"""
+
+import subprocess
+import sys
+from pathlib import Path
+
+import entity_stublib
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+TRACKED = ("homeassistant", "voluptuous", "nsh")
+
+
+def _scoped_faces() -> list[str]:
+    """The HA/voluptuous faces every file scopes on (the nsh.* names are modules)."""
+    return [name for name in entity_stublib.ENTITIES_STUB_NAMES if not name.startswith("nsh.")]
+
+
+def _run(*paths: str) -> subprocess.CompletedProcess:
+    return subprocess.run(
+        [
+            sys.executable,
+            "-m",
+            "pytest",
+            *paths,
+            "-q",
+            "--no-header",
+            "-p",
+            "no:cacheprovider",
+        ],
+        cwd=REPO_ROOT,
+        capture_output=True,
+        text=True,
+        timeout=300,
+    )
+
+
+class TestStubFaceGuard:
+    """Mechanism 1: a partially popped face set must be repaired, not trusted."""
+
+    def test_install_provides_every_face_a_file_scopes_on(self):
+        """A face listed for scoping but never installed is a file-order trap."""
+        snapshot = entity_stublib.snapshot_modules()
+        try:
+            entity_stublib.install()
+
+            missing = [name for name in _scoped_faces() if name not in sys.modules]
+
+            assert missing == []
+        finally:
+            entity_stublib.restore_modules(snapshot)
+
+    def test_a_complete_face_set_is_left_alone(self):
+        """The guard must still be an idempotence guard, not a rebuild."""
+        snapshot = entity_stublib.snapshot_modules()
+        try:
+            entity_stublib.install()
+            before = {name: sys.modules[name] for name in _scoped_faces()}
+
+            entity_stublib.install()
+
+            assert {name: sys.modules[name] for name in _scoped_faces()} == before
+        finally:
+            entity_stublib.restore_modules(snapshot)
+
+    def test_a_sentinel_left_behind_does_not_hide_a_missing_face(self):
+        """The reported poisoning: media_player present, voluptuous gone."""
+        snapshot = entity_stublib.snapshot_modules()
+        try:
+            entity_stublib.install()
+            kept = sys.modules["homeassistant.components.media_player"]
+            del sys.modules["voluptuous"]
+
+            entity_stublib.install()
+
+            assert "voluptuous" in sys.modules
+            # the rebuild is wholesale, so the sentinel is a real face again
+            assert sys.modules["homeassistant.components.media_player"] is not None
+            assert kept is not None
+        finally:
+            entity_stublib.restore_modules(snapshot)
+
+
+class TestTeardownLeaks:
+    """Mechanism 2: nothing a file created or replaced may survive its teardown."""
+
+    def test_teardown_removes_the_modules_a_file_loaded(self):
+        snapshot = entity_stublib.snapshot_modules()
+        try:
+            entity_stublib.install()
+            entity_stublib.load_entity_modules()
+            created = [
+                name
+                for name in sys.modules
+                if name.startswith("nsh.") and name not in snapshot
+            ]
+            assert created, "premise: loading the entity modules creates nsh.* modules"
+
+            entity_stublib.restore_modules(snapshot)
+
+            assert [name for name in sys.modules if name.startswith("nsh.")] == [
+                name for name in snapshot if name.startswith("nsh.")
+            ]
+            assert {
+                name: sys.modules.get(name)
+                for name in snapshot
+                if name.split(".", 1)[0] in TRACKED
+            } == snapshot
+        finally:
+            entity_stublib.restore_modules(snapshot)
+
+    def test_a_reloaded_module_holds_the_current_stub_objects(self):
+        """The stale-object failure, exactly: diagnostics vs the redact helper."""
+        snapshot = entity_stublib.snapshot_modules()
+        try:
+            entity_stublib.install()
+            stale_module = entity_stublib.load_entity_modules()["diagnostics"]
+            stale = stale_module.async_redact_data
+            entity_stublib.restore_modules(snapshot)  # ...the file ends
+
+            # ...the next file installs its own faces: a partial pop is how that
+            # actually looked, and it is what forces the rebuild
+            entity_stublib.install()
+            del sys.modules["homeassistant.helpers.redact"]
+            entity_stublib.install()
+            fresh_modules = entity_stublib.load_entity_modules()
+
+            current = sys.modules["homeassistant.helpers.redact"].async_redact_data
+            assert stale is not current, "premise: the rebuild created a new helper"
+            assert fresh_modules["diagnostics"].async_redact_data is current
+            # ...and the stale module is not the one sitting in the import cache
+            assert sys.modules["nsh.diagnostics"] is fresh_modules["diagnostics"]
+        finally:
+            entity_stublib.restore_modules(snapshot)
+
+
+@pytest.mark.parametrize(
+    "paths",
+    [
+        # the reported reproduction, cut to the two files that trigger it
+        ("tests/test_standalone_setup.py", "tests/test_init.py::TestImportStep"),
+        # the other failure mode: a file that loads the entity modules before a
+        # file that compares them against freshly installed faces
+        (
+            "tests/test_standalone_setup.py",
+            "tests/test_config_flow_protocol.py",
+            "tests/test_init.py",
+            "tests/test_command_routing.py::TestAdbCommandService",
+            "tests/test_command_routing.py::TestFaithfulHaStubs",
+            "tests/test_command_routing.py::TestServiceRegistryKey",
+        ),
+    ],
+)
+def test_arbitrary_file_orders_pass(paths):
+    """Order is the thing under test, so it is executed, not reasoned about."""
+    result = _run(*paths)
+
+    assert result.returncode == 0, result.stdout[-3000:]