Scope core.* module stubs to the test, not the module (#1513)

Three test files (test_auth_regressions, test_auth_event_loop,
test_null_owner_gates) install stubs for core.database / core.auth /
src.endpoint_resolver at module-import time, so they outlive the
file and are still present in sys.modules when later-collected test
files try to import the real modules. The stubs are minimal (a
handful of MagicMock attrs) so the import chain that follows fails
with ImportError on the very next real import.

test_companion_pairing also leaks, with a twist: its _DBStub
subclass returns a MagicMock for *any* attribute including dunders,
so the next test that does `from core.database import *` reads
`__all__` as a MagicMock and dies with 'Item in __all__ must be
str, not MagicMock'.

Move the stub installation into an autouse fixture per file and
register each stub with monkeypatch.setitem so sys.modules is
restored to its pre-test state on teardown. Tighten _DBStub to
refuse dunder names so __all__ stays undefined. _CAPTURED is
cleared per test so the mint-token assertions see a fresh dict.

Before: 3 test files fail at collection time (test_chat_image_routing,
test_context_compactor, test_webhook_ssrf_resilience). After: 0
collection errors. 1365/1370 pass, 1 skip, 4 unrelated pre-existing
failures (verified against origin/main baseline).

Out of scope: test_task_scheduler_session_delivery::
test_session_delivery_survives_empty_database also fails in the
full suite due to order-dependent state from a different test
file. That's a separate leak with a different root cause.

tests/test_companion_pairing.py

@@ -37,27 +37,36 @@ def _get_db_session():
 
 # core/__init__ pulls in models/session_manager which import many ORM names from
 # core.database; under conftest's sqlalchemy stubs the real module can't load.
-# A __getattr__ module resolves ANY requested name to a MagicMock, while keeping
-# our real get_db_session/ApiToken for the mint test.
+# A __getattr__ module resolves any non-dunder name to a MagicMock, while keeping
+# our real get_db_session/ApiToken for the mint test. Dunder names (e.g. __all__)
+# are NOT auto-resolved — the next test file does `from core.database import *`,
+# which would otherwise see a MagicMock where a list-of-str is required.
 class _DBStub(types.ModuleType):
     def __getattr__(self, name):  # noqa: D401
+        if name.startswith("__"):
+            raise AttributeError(name)
         return MagicMock()
 
 
 _db = _DBStub("core.database")
 _db.get_db_session = _get_db_session
 _db.ApiToken = _ApiToken
-sys.modules["core.database"] = _db  # overwrite any minimal stub from a sibling test
 
-for _name, _attrs in {
-    "core.auth": {"AuthManager": MagicMock()},
-    "src.endpoint_resolver": {"build_chat_url": (lambda u: u)},
-}.items():
-    if _name not in sys.modules:
-        _mm = types.ModuleType(_name)
-        for _k, _v in _attrs.items():
-            setattr(_mm, _k, _v)
-        sys.modules[_name] = _mm
+
+@pytest.fixture(autouse=True)
+def _companion_pairing_stubs(monkeypatch):
+    monkeypatch.setitem(sys.modules, "core.database", _db)
+    for _name, _attrs in {
+        "core.auth": {"AuthManager": MagicMock()},
+        "src.endpoint_resolver": {"build_chat_url": (lambda u: u)},
+    }.items():
+        if _name not in sys.modules:
+            _mm = types.ModuleType(_name)
+            for _k, _v in _attrs.items():
+                setattr(_mm, _k, _v)
+            sys.modules[_name] = _mm
+        monkeypatch.setitem(sys.modules, _name, sys.modules[_name])
+
 
 from fastapi import HTTPException  # noqa: E402