refactor(tests): one definition each for the copied fixtures and fakes (#2825, milestone 296 area 1)

The shape ledger showed the same test scaffolding defined over and over:
_bind_user x12 (byte-identical), _dispose_engine x10 in three wordings,
_no_supersession x3, _make_mock_session x7 in three subsets, a get-or-create
User helper x2 (+3 inlined), and fifteen hand-rolled MagicMock note factories
each re-explaining the same "an auto-MagicMock attribute is truthy" hazard
(note 2109).

Now: conftest.py carries _bind_user / _dispose_engine / _no_supersession as
opt-in fixtures (pytestmark = usefixtures(...) per module, so unit tests pay
nothing), and tests/helpers.py carries make_mock_session(), ensure_user() and
fake_note(**attrs) — the hazard documented once, real values on every
attribute the product reads. Call sites were rewritten by AST so titles with
dashes and commas survived; the three SimpleNamespace _note stand-ins that
only feed a single function stay local.

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

tests/test_mcp_tool_search.py

@@ -1,12 +1,13 @@
 """search tool — proves the tool pattern (context + service call + dict shape).
 
 Service call is mocked; no DB needed."""
-from unittest.mock import AsyncMock, MagicMock, patch
+from unittest.mock import AsyncMock, patch
 
 import pytest
 
 from scribe.mcp._context import _user_id_ctx
 from scribe.mcp.tools.search import search
+from tests.helpers import fake_note
 
 
 @pytest.fixture(autouse=True)
@@ -17,22 +18,6 @@ def _reset_user_ctx():
     _user_id_ctx.reset(token)
 
 
-def _fake_note(*, id: int, title: str, body: str = "",
-               tags: list[str] | None = None, is_task: bool = False,
-               user_id: int = 7) -> MagicMock:
-    note = MagicMock()
-    note.id = id
-    note.title = title
-    note.body = body
-    note.tags = tags or []
-    note.is_task = is_task
-    # A real int, matching the bound caller by default: results compare it to
-    # decide whether to attach a shared/owner marker, and an auto-MagicMock would
-    # read as "someone else's" and send the tool looking up a username.
-    note.user_id = user_id
-    return note
-
-
 @pytest.mark.asyncio
 async def test_fable_search_raises_without_context():
     with pytest.raises(RuntimeError, match="no MCP user context"):
@@ -42,8 +27,7 @@ async def test_fable_search_raises_without_context():
 @pytest.mark.asyncio
 async def test_fable_search_returns_repackaged_results():
     _user_id_ctx.set(7)
-    fake = _fake_note(id=1, title="kafka rebalance", body="HPA details",
-                      tags=["ops"], is_task=False)
+    fake = fake_note(id=1, title="kafka rebalance", is_task=False, body="HPA details", tags=["ops"])
     with patch(
         "scribe.mcp.tools.search.semantic_search_notes",
         AsyncMock(return_value=[(0.93, fake)]),
@@ -65,7 +49,7 @@ async def test_fable_search_returns_repackaged_results():
 async def test_fable_search_body_is_truncated_to_240_chars():
     _user_id_ctx.set(7)
     long_body = "x" * 500
-    fake = _fake_note(id=1, title="t", body=long_body)
+    fake = fake_note(id=1, title="t", body=long_body)
     with patch(
         "scribe.mcp.tools.search.semantic_search_notes",
         AsyncMock(return_value=[(0.5, fake)]),