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_integration_pgvector_search.py

@@ -16,13 +16,13 @@ import pytest
 import pytest_asyncio
 from sqlalchemy import delete
 
-from scribe.models import async_session, engine
+from scribe.models import async_session
 from scribe.models.embedding import EMBEDDING_DIM, NoteEmbedding
 from scribe.models.note import Note
 from scribe.models.user import User
 from scribe.services.embeddings import semantic_search_notes
 
-pytestmark = pytest.mark.integration
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
 
 
 def _vec(*nonzero_first):
@@ -45,13 +45,6 @@ def _emb(note_id, user_id, chunk_index, vec):
     )
 
 
-@pytest_asyncio.fixture(autouse=True)
-async def _dispose_engine():
-    # Per-loop pool: dispose after each test (see test_integration_db_maintenance).
-    yield
-    await engine.dispose()
-
-
 @pytest_asyncio.fixture
 async def seeded():
     """Insert a user + a near and a far note with hand-crafted embeddings.