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_note_usage.py

@@ -8,22 +8,9 @@ before this it surfaced snippets while leaving no trace anywhere.
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
-import pytest_asyncio
 
 
-@pytest.fixture(autouse=True)
-def _no_supersession():
-    """The auto-inject menu now asks which of its lines are superseded (#278).
-
-    That is a real database call on a path these tests exercise without one.
-    Stubbed to "nothing superseded" — the ordinary state — rather than hidden
-    behind a try/except in the product, which would make the code lie about
-    what it does. The label's own behaviour is covered in
-    tests/test_supersession_ranking.py.
-    """
-    with patch("scribe.services.plugin_context.superseded_ids",
-               AsyncMock(return_value=set())):
-        yield
+pytestmark = pytest.mark.usefixtures("_no_supersession")
 
 
 from scribe.services import note_usage
@@ -33,19 +20,7 @@ from scribe.services.note_usage import (
     record_surfaced,
     usage_for_notes,
 )
-
-
-def _note(nid, title, user_id=1):
-    n = MagicMock()
-    n.id, n.title, n.user_id = nid, title, user_id
-    n.note_type = "snippet"
-    # See the same note in test_write_path_trigger: an auto-created mock on
-    # `.data` is truthy and would leak into a rendered menu line (#2244).
-    n.data = None
-    # And on `.is_task`, which the kind marker reads FIRST — truthy there makes
-    # every one of these snippets read as a task (#2246).
-    n.is_task, n.task_kind = False, "work"
-    return n
+from tests.helpers import fake_note
 
 
 # --- recording ------------------------------------------------------------
@@ -207,7 +182,7 @@ async def test_auto_inject_records_what_survived_the_margin_gate():
     two numbers must not silently mean different things per surface."""
     from scribe.services import plugin_context
 
-    hits = [(0.90, _note(1, "kept")), (0.40, _note(2, "cut by the margin gate"))]
+    hits = [(0.90, fake_note(id=1, title="kept", user_id=1, note_type="snippet")), (0.40, fake_note(id=2, title="cut by the margin gate", user_id=1, note_type="snippet"))]
     with (
         patch.object(
             plugin_context,
@@ -271,13 +246,6 @@ def test_every_getter_that_can_be_surfaced_also_records_a_pull():
 # and split the chain so a failure names its half.
 
 
-@pytest_asyncio.fixture
-async def _dispose_engine():
-    from scribe.models import engine
-    yield
-    await engine.dispose()
-
-
 async def _purge(note_id: int) -> None:
     from sqlalchemy import delete