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_forge_webhook.py

@@ -21,6 +21,7 @@ import pytest_asyncio
 
 from scribe.routes.webhooks import delivered_signature, push_facts, signature_ok
 from scribe.services.snippets import _path_touches
+from tests.helpers import ensure_user
 
 SECRET = "wh-secret"
 HEAD = "e" * 40
@@ -122,32 +123,17 @@ def test_route_is_registered_and_unauthenticated_by_design():
 
 # --- integration: the flag lands and clears on real Postgres -----------------
 
-@pytest_asyncio.fixture
-async def _dispose_engine():
-    from scribe.models import engine
-    yield
-    await engine.dispose()
-
 
 @pytest_asyncio.fixture
 async def seeded(_dispose_engine):
     """User + project + binding + two verified snippets + one unverified."""
-    from sqlalchemy import select
-
     from scribe.models import async_session
     from scribe.models.project import Project
-    from scribe.models.user import User
     from scribe.services import snippets as svc
     from scribe.services.repo_bindings import set_binding
 
     async with async_session() as s:
-        user = (
-            await s.execute(select(User).where(User.username == "webhook_itest"))
-        ).scalar_one_or_none()
-        if user is None:
-            user = User(username="webhook_itest")
-            s.add(user)
-            await s.flush()
+        user = await ensure_user(s, "webhook_itest")
         project = Project(user_id=user.id, title="Widget")
         s.add(project)
         await s.flush()