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_snippet_merge_provenance.py

@@ -6,26 +6,20 @@ only in the trash — and only for someone who already knew to go looking. So th
 survivor carries `merged_from`, written to the body and the indexed mirror from
 one value, and ordinary edits must carry it forward rather than erase it.
 """
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, patch
 
 import pytest
 
 from scribe.services import snippets as s
+from tests.helpers import fake_snippet
 
 
 def _snippet(id=1, owner=9, body=None, data=None, tags=None):
-    n = MagicMock()
-    n.id = id
-    n.user_id = owner
-    n.title = "formatDuration — humanize a millisecond count"
-    n.body = body if body is not None else s.compose_body(code="x = 1", language="ts")
-    n.tags = tags if tags is not None else ["ts", "snippet"]
-    n.note_type = "snippet"
-    n.deleted_at = None
-    # Explicitly None — snippet_fields prefers `data` when truthy, and an
-    # auto-MagicMock attribute is truthy (note 2109).
-    n.data = data
-    return n
+    return fake_snippet(
+        id=id, user_id=owner, title="formatDuration — humanize a millisecond count",
+        body=body if body is not None else s.compose_body(code="x = 1", language="ts"),
+        tags=tags if tags is not None else ["ts", "snippet"], data=data,
+    )
 
 
 async def _run_merge(target, sources, source_ids):