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_milestones.py

@@ -3,17 +3,12 @@ from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 
-from scribe.mcp._context import _user_id_ctx
 from scribe.mcp.tools.milestones import (
     list_milestones, get_milestone, create_milestone, update_milestone,
 )
 
 
-@pytest.fixture(autouse=True)
-def _bind_user():
-    token = _user_id_ctx.set(7)
-    yield
-    _user_id_ctx.reset(token)
+pytestmark = pytest.mark.usefixtures("_bind_user")
 
 
 def _fake_ms(**overrides) -> MagicMock: