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_config_flow_ux.py

@@ -26,6 +26,7 @@ from pathlib import Path
 from types import SimpleNamespace
 from unittest.mock import AsyncMock
 
+import entity_stublib
 import pytest
 import voluptuous_mirror as vol_mirror
 from conftest import _SHIELD_DIR, _load_module
@@ -331,17 +332,12 @@ def scoped_stubs():
     another file left behind from shadowing the faithful voluptuous mirror
     this file's schema assertions depend on.
     """
-    snapshot = {name: sys.modules.get(name) for name in _STUB_NAMES}
+    snapshot = entity_stublib.snapshot_modules()
     for name in _STUB_NAMES:
         sys.modules.pop(name, None)
     _install_stubs()
     yield
-    for name in _STUB_NAMES:
-        original = snapshot[name]
-        if original is None:
-            sys.modules.pop(name, None)
-        else:
-            sys.modules[name] = original
+    entity_stublib.restore_modules(snapshot)
 
 
 def _form_module():