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_services_retrieval_telemetry.py

@@ -7,7 +7,6 @@ integration test against real Postgres.
 from types import SimpleNamespace
 
 import pytest
-import pytest_asyncio
 
 from scribe.services.retrieval_telemetry import (
     _build_payload,
@@ -72,13 +71,6 @@ def test_record_retrieval_without_event_loop_is_safe():
 # ─── persistence (integration) ───────────────────────────────────────────────
 
 
-@pytest_asyncio.fixture
-async def _dispose_engine():
-    from scribe.models import engine
-    yield
-    await engine.dispose()
-
-
 @pytest.mark.integration
 @pytest.mark.asyncio
 async def test_insert_retrieval_log_roundtrip(_dispose_engine):