fix(console): preserve cs=None fallback in /v1/api/models placeholder

Copilot review feedback on #500.  The original
``list_available_models`` had an implicit cs=None branch where the
placeholder still advertised ``registry.default`` (filtered against
enabled rows) when ``app.state.config_store`` was None but
``coord_registry`` was bound — useful in the rare degraded state
where lifespan wired the registry but the ConfigStore failed to
initialise.  The PR #500 refactor accidentally dropped that branch:
the helper requires a config_store, so the cs=None case fell out as
"blank coordinator default".

Add an explicit ``elif coord_registry is not None`` branch that
mirrors the helper's tier 3 with the placeholder's enabled-rows
filter applied.  New test exercises this path by passing
``config_store=False`` to the test fixture.

tests/test_console_available_models.py

@@ -88,13 +88,15 @@ def _make_client(
     *,
     settings: dict[str, str] | None = None,
     registry_default: str = "",
+    config_store: bool = True,
 ) -> TestClient:
     app = Starlette(
         routes=[Route("/v1/api/models", list_available_models)],
         middleware=[Middleware(_AuthMiddleware)],
     )
     app.state.auth_storage = storage
-    app.state.config_store = _FakeConfigStore(dict(settings or {}))
+    if config_store:
+        app.state.config_store = _FakeConfigStore(dict(settings or {}))
     # ``coord_registry`` is always set in production after lifespan
     # startup; mirror that here.  ``has_alias`` answers from the same
     # enabled-rows set the handler filters against.
@@ -219,6 +221,22 @@ def test_coordinator_skips_registry_default_when_alias_disabled(
     assert body["coordinator_default_alias"] == ""
 
 
+def test_coordinator_falls_back_to_registry_default_when_config_store_missing(
+    storage: SQLiteBackend,
+) -> None:
+    """Edge case from PR #500 review: lifespan can leave
+    ``app.state.config_store`` as None (e.g. a startup exception) while
+    ``coord_registry`` still binds successfully.  The placeholder must
+    still advertise ``registry.default`` (filtered against enabled rows)
+    rather than going blank — otherwise the home composer is uselessly
+    empty in a degraded-but-recoverable state."""
+    _seed_model(storage, definition_id="m1", alias="primary")
+    body = _get_models(_make_client(storage, registry_default="primary", config_store=False))
+    assert body["default_alias"] == ""
+    assert body["coordinator_default_alias"] == "primary"
+    assert body["judge_default_alias"] == "primary"
+
+
 # ---------------------------------------------------------------------------
 # Judge resolution
 # ---------------------------------------------------------------------------

turnstone/console/server.py

@@ -1699,6 +1699,14 @@ async def list_available_models(request: Request) -> JSONResponse:
             registry=coord_registry,
             alias_filter=lambda a: a in enabled_aliases,
         )
+    elif coord_registry is not None:
+        # ConfigStore failed lifespan but coord_registry is still bound —
+        # fall through to ``registry.default`` (filtered) so the home
+        # composer isn't blank.  Mirrors the helper's tier 3 with the
+        # placeholder's enabled-rows filter applied.
+        registry_default = getattr(coord_registry, "default", "") or ""
+        if registry_default in enabled_aliases:
+            coordinator_default_alias = registry_default
     # Judge falls back to the resolved coordinator alias when
     # ``judge.model`` is empty *or* not a registered alias — judge.model
     # is alias-only (matches IntentJudge.__init__), so an unknown value is