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_task_scheduler_session_delivery.py

@@ -14,15 +14,14 @@ from sqlalchemy.orm import sessionmaker
 from core.database import Base, Session as DbSession
 from src.task_scheduler import TaskScheduler
 
-# TEMPORARY ISOLATION WORKAROUND — remove once test_null_owner_gates.py is
-# refactored to use a fixture-scoped stub instead of module-level sys.modules
-# patching.  When collected after test_null_owner_gates (alphabetical order),
-# core.database is already a stub whose Base attribute is a MagicMock, so
-# Base.metadata.create_all() below does nothing and the assertions fail.
-# The test passes correctly in isolation:
-#   pytest tests/test_task_scheduler_session_delivery.py   → 1 passed
-# Full-suite baseline before this PR:  9 failed, 345 passed  (pre-upstream-pull)
-# Full-suite after this PR:            1 failed, 495 passed, 1 skipped
+# This test needs the real core.database (real SQLAlchemy Base/ChatMessage).
+# test_null_owner_gates.py no longer leaks its stubs (per-test fixture cleanup
+# since PR #1513), but several other files still install core.database stubs
+# at module level without teardown (test_model_routes, test_companion_readonly,
+# test_endpoint_probing, test_vault_password_not_in_argv).  When any of those
+# are collected before us, core.database is a stub and Base is a MagicMock.
+# Skip in that case — the test passes correctly in isolation or when collected
+# before the stubbing files.
 if type(Base).__name__ == "MagicMock":
     pytest.skip("core.database is stubbed — run this file in isolation", allow_module_level=True)