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_shape_classify.py

@@ -9,7 +9,7 @@ import pytest
 import pytest_asyncio
 from sqlalchemy import select
 
-from scribe.models import async_session, engine
+from scribe.models import async_session
 from scribe.models.code_shape import CodeShape
 from scribe.models.project import Project
 from scribe.models.user import User
@@ -19,8 +19,9 @@ from scribe.services.shape_ledger import (
     snippet_consumers,
     sync_repo_shapes,
 )
+from tests.helpers import ensure_user
 
-pytestmark = pytest.mark.integration
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
 
 REPO = "git.example.com/alice/widget"
 SHAPES = [
@@ -31,39 +32,14 @@ SHAPES = [
 ]
 
 
-@pytest_asyncio.fixture(autouse=True)
-async def _dispose_engine():
-    """Dispose the app's module-level engine after each test.
-
-    The engine pools asyncpg connections per event loop, but pytest-asyncio runs
-    each test on a fresh loop — so without this, test 2 gets handed test 1's
-    connection bound to a now-dead loop ("Future attached to a different loop").
-    Disposing in the test's own loop teardown clears the pool cleanly.
-    """
-    yield
-    await engine.dispose()
-
-
-async def _user(session, username: str) -> User:
-    existing = (
-        await session.execute(select(User).where(User.username == username))
-    ).scalar_one_or_none()
-    if existing is not None:
-        return existing
-    user = User(username=username)
-    session.add(user)
-    await session.flush()
-    return user
-
-
 @pytest_asyncio.fixture
 async def seeded():
     """Owner + outsider, a project with a synced 4-shape ledger, one snippet."""
     from scribe.services import snippets as snippets_svc
 
     async with async_session() as s:
-        owner = await _user(s, "classify_owner")
-        other = await _user(s, "classify_other")
+        owner = await ensure_user(s, "classify_owner")
+        other = await ensure_user(s, "classify_other")
         project = Project(user_id=owner.id, title="Classify target")
         s.add(project)
         await s.flush()