refactor(tests): per-model fakes, FakeMCP and session mocks come from tests/helpers (#2825, milestone 296 area 1, batch 2)

Second pass over the tests/ ledger after bbee0d0. fake_record(**attrs) is the
one MagicMock-with-real-attributes builder (to_dict mirrors them; the
note-2109 hazard documented once); fake_note/fake_task/fake_snippet/
fake_project/fake_milestone/fake_system/fake_rulebook/fake_topic/fake_rule
carry each model's ordinary defaults on top of it, replacing 14 per-file
factories (two rulebook trios in tool-vs-service wordings, _fake_task, _fake_ms,
_fake_project, _plan_note, _fake_snippet, two _snippet adapters now one-liners
over fake_snippet). FakeMCP replaces the five closure-over-a-list registrar
fakes (+ _Recorder); loc() and design_token_stub() replace the paired _loc /
_token / _T stand-ins; every hand-built async_session mock (9 helper defs and
14 inline copies) now starts from make_mock_session(). Call sites rewritten by
AST with each file's former defaults made explicit, so behaviour is unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_version_pinning_pin_unpin.py

@@ -1,16 +1,15 @@
 """Tests for manual pin / unpin on note versions."""
 from unittest.mock import AsyncMock, MagicMock, patch
+from tests.helpers import make_mock_session
 
 
 def _mock_session_for_version(mock_version):
-    mock_session = AsyncMock()
+    mock_session = make_mock_session()
     result = MagicMock()
     result.scalars.return_value.first.return_value = mock_version
     mock_session.execute = AsyncMock(return_value=result)
     mock_session.commit = AsyncMock()
     mock_session.refresh = AsyncMock()
-    mock_session.__aenter__ = AsyncMock(return_value=mock_session)
-    mock_session.__aexit__ = AsyncMock(return_value=False)
     return mock_session
 
 
@@ -92,14 +91,12 @@ async def test_pin_version_rejects_overlong_label():
 
 
 async def test_pin_version_returns_none_when_not_found():
-    mock_session = AsyncMock()
+    mock_session = make_mock_session()
     result = MagicMock()
     result.scalars.return_value.first.return_value = None
     mock_session.execute = AsyncMock(return_value=result)
     mock_session.commit = AsyncMock()
     mock_session.refresh = AsyncMock()
-    mock_session.__aenter__ = AsyncMock(return_value=mock_session)
-    mock_session.__aexit__ = AsyncMock(return_value=False)
 
     with patch(
         "scribe.services.version_pinning.async_session",