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_retrieval_scopes.py

@@ -15,38 +15,10 @@ shared record would be findable by wording and invisible by meaning.
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
+from tests.helpers import fake_note
 
 
-@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-    """The auto-inject menu now asks which of its lines are superseded (#278).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-    Stubbed to "nothing superseded" — the ordinary state — rather than hidden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
-
-
-
-def _note(id=1, user_id=7, title="A note", note_type="note",
-          is_task=False, task_kind="work"):
-    n = MagicMock()
-    n.id = id
-    n.user_id = user_id
-    n.title = title
-    n.body = "body"
-    n.tags = []
-    # Real values, not auto-attributes: the menu reads these to label each line,
-    # and a MagicMock is truthy — every record would render as a task (note 2109).
-    n.is_task = is_task
-    n.task_kind = task_kind
-    n.note_type = note_type
-    return n
+pytestmark = pytest.mark.usefixtures("_no_supersession")
 
 
 @pytest.fixture
@@ -148,7 +120,7 @@ async def test_injected_menu_attributes_another_users_note():
     menu line is the only provenance the agent sees, so it has to name them."""
     from scribe.services import plugin_context
 
-    mine, theirs = _note(1, user_id=7, title="Mine"), _note(2, user_id=9, title="Theirs")
+    mine, theirs = fake_note(id=1, title="Mine", user_id=7), fake_note(id=2, title="Theirs", user_id=9)
     with patch.object(plugin_context, "semantic_search_notes",
                       AsyncMock(return_value=[(0.9, theirs), (0.88, mine)])), \
          patch.object(plugin_context, "get_autoinject_config",
@@ -176,11 +148,11 @@ async def test_injected_menu_labels_the_record_kind():
     from scribe.services import plugin_context
 
     hits = [
-        (0.92, _note(1, title="debounce — rate-limit a callback", note_type="snippet")),
-        (0.91, _note(2, title="Release checklist", note_type="process")),
-        (0.90, _note(3, title="Auth token expiry", is_task=True, task_kind="issue")),
-        (0.89, _note(4, title="Ship the drafter", is_task=True)),
-        (0.88, _note(5, title="Why we dropped CalDAV")),
+        (0.92, fake_note(id=1, title="debounce — rate-limit a callback", note_type="snippet")),
+        (0.91, fake_note(id=2, title="Release checklist", note_type="process")),
+        (0.90, fake_note(id=3, title="Auth token expiry", is_task=True, task_kind="issue")),
+        (0.89, fake_note(id=4, title="Ship the drafter", is_task=True)),
+        (0.88, fake_note(id=5, title="Why we dropped CalDAV")),
     ]
     with patch.object(plugin_context, "semantic_search_notes",
                       AsyncMock(return_value=hits)), \