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_prune.py

@@ -10,7 +10,7 @@ async def test_create_version_prune_sql_filters_to_unpinned():
     """The DELETE statement issued by create_version's prune step must
     include `pin_kind IS NULL` in the inner SELECT so pinned versions
     aren't counted toward MAX_VERSIONS and can't be pruned by it."""
-    mock_session = AsyncMock()
+    mock_session = make_mock_session()
     select_result = MagicMock()
     # No prior version → skips the throttle/dedupe early-return paths and
     # proceeds straight to insert + prune.
@@ -18,8 +18,6 @@ async def test_create_version_prune_sql_filters_to_unpinned():
     mock_session.add = MagicMock()
     mock_session.commit = AsyncMock()
     mock_session.refresh = AsyncMock()
-    mock_session.__aenter__ = AsyncMock(return_value=mock_session)
-    mock_session.__aexit__ = AsyncMock(return_value=False)
 
     captured_sql: list[str] = []
 
@@ -48,10 +46,8 @@ async def test_create_version_prune_sql_filters_to_unpinned():
 async def test_prune_auto_pins_filters_to_auto_kind():
     """prune_auto_pins must filter to pin_kind='auto' so manual pins and
     rolling rows aren't touched, and must bind MAX_AUTO_PINS as the OFFSET."""
-    mock_session = AsyncMock()
+    mock_session = make_mock_session()
     mock_session.commit = AsyncMock()
-    mock_session.__aenter__ = AsyncMock(return_value=mock_session)
-    mock_session.__aexit__ = AsyncMock(return_value=False)
 
     captured_sql: list[str] = []
     captured_params: list[dict] = []