fix(config_flow): clearing an entity field no longer fails validation (t47 C)

t46 F1, reproduced against real Home Assistant: submitting the options page with a
cleared entity field (script_turn_on='') produced "Schema validation failed at
script_turn_on". HA validates the submitted page against the schema the flow last
returned, and real EntitySelector rejects BOTH '' and None — "Entity  is neither a
valid entity ID nor a valid UUID" (verified directly against homeassistant's
EntitySelector). Clearing therefore depended on the frontend OMITTING the key, on
every entity field of the page, not only the script ones t44 added.

_ClearingEntitySelector subclasses EntitySelector and returns '' for None or an
empty/whitespace string, delegating everything else to the parent — so a real entity
id is still domain-checked, and the class is still an EntitySelector: its serialized
form is byte-identical to the real selector's
({"selector": {"entity": {"domain": ["script"], "multiple": false, "reorder": false}}}),
so the frontend keeps rendering an entity picker.

The step also normalizes a submitted None to '' before it acts: an explicit empty
value clears the setting (the script dict drops it, the patched options store '' which
every reader already treats as unset), while an OMITTED key still means "the page did
not touch it" for the patched options — and still means "cleared" for the script dict,
which is rebuilt from the page (t44's editor-is-the-truth rule, unchanged).

The test harness now models the trap instead of hiding it: EntitySelector is a CLASS
(entity_stublib.EntitySelectorStub) that rejects ''/None like real HA, defined once and
shared by all five stub installers through install_selector_faces(), so the stubs
cannot drift apart.

Tests (tests/test_config_flow_ux.py::TestClearingAnEntityField, 8): the schema accepts
omitted/None/'' while passing a real entity id through; the plain selector is pinned as
rejecting the same values (the trap itself); '' and None each clear a script AND a
pre-existing entity field (jellyfin, the older instance of the pattern) while
untouched fields survive; an omitted script field is cleared; an omitted entity field
is kept; and a prefill→submit round trip changes nothing.

RED PROOF: with the five selector sites switched back to the plain EntitySelector,
exactly the five tolerance tests fail (5 failed, 4 passed) and the rest of the file
stays green; restored, 70 pass. Real-HA probe run separately (see the task output):
strict rejects '' and None, the subclass accepts both and returns '', and serialization
is unchanged.

Verified: 423 passed / 2 skipped, ruff clean, drift engine exit 0 under real HA.

tests/test_config_flow_pairing.py

@@ -127,11 +127,7 @@ def _install_import_stubs() -> None:
     helpers.device_registry = device_registry
     sys.modules["homeassistant.helpers.device_registry"] = device_registry
     selector = types.ModuleType("homeassistant.helpers.selector")
-    selector.EntitySelector = lambda *a, **k: None
-    selector.EntitySelectorConfig = lambda *a, **k: ""
-    selector.SelectSelector = lambda *a, **k: None
-    selector.SelectSelectorConfig = lambda **k: ""
-    selector.SelectSelectorMode = types.SimpleNamespace(DROPDOWN="dropdown")
+    entity_stublib.install_selector_faces(selector)
     sys.modules["homeassistant.helpers.selector"] = selector