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_systems.py

@@ -2,6 +2,7 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
+from tests.helpers import fake_note
 
 
 def _fake_system(sid=1, name="Reader", project_id=5):
@@ -88,12 +89,6 @@ async def test_untagged_hint_names_the_projects_systems():
     assert "create_system" in hint
 
 
-def _fake_note(title):
-    n = MagicMock()
-    n.title = title
-    return n
-
-
 @pytest.mark.asyncio
 async def test_untagged_hint_zero_systems_prompts_first_create_and_fails_open():
     from scribe.mcp.tools.systems import untagged_systems_hint
@@ -120,7 +115,7 @@ async def test_untagged_hint_escalates_in_a_mature_zero_systems_project():
     concrete deliverable, because that is the property separating the nudges
     that convert from the prose that doesn't."""
     from scribe.mcp.tools.systems import untagged_systems_hint
-    recent = [_fake_note("Fix scrape retry backoff"), _fake_note("Worker pool sizing")]
+    recent = [fake_note(title="Fix scrape retry backoff"), fake_note(title="Worker pool sizing")]
     with patch("scribe.mcp.tools.systems.systems_svc") as svc, \
          patch("scribe.mcp.tools.systems.notes_svc") as notes:
         svc.list_systems = AsyncMock(return_value=[])