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_pattern_coverage.py

@@ -23,6 +23,7 @@ from scribe.services.coverage import (
     shapes_from_archive,
 )
 from scribe.services.shape_ledger import location_covers
+from tests.helpers import ensure_user
 
 # --- unit: the definition extractor (shared vectors with the hook) -----------
 
@@ -204,32 +205,16 @@ def _selector(tar_bytes: bytes):
     return ForgeSelector((_forge(tar_bytes),))
 
 
-@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 snippets that cover 2 of TREE's 4 shapes."""
-    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 == "coverage_itest"))
-        ).scalar_one_or_none()
-        if user is None:
-            user = User(username="coverage_itest")
-            s.add(user)
-            await s.flush()
+        user = await ensure_user(s, "coverage_itest")
         project = Project(user_id=user.id, title="Widget")
         s.add(project)
         await s.flush()
@@ -396,9 +381,7 @@ async def test_explicit_refresh_names_its_failures(seeded):
     an agent mid-task must learn WHY nothing measured — 'None' is exactly the
     stranding the button-only path caused."""
     from scribe.models import async_session
-    from scribe.models.user import User
     from scribe.services.coverage import refresh_for_caller
-    from sqlalchemy import select
 
     uid, pid = seeded["uid"], seeded["pid"]
     # The owner has no forge connection rows → the error names the fix.
@@ -408,13 +391,7 @@ async def test_explicit_refresh_names_its_failures(seeded):
 
     # A stranger gets not-found/no-write, never a measurement.
     async with async_session() as s:
-        other = (await s.execute(
-            select(User).where(User.username == "coverage_outsider")
-        )).scalar_one_or_none()
-        if other is None:
-            other = User(username="coverage_outsider")
-            s.add(other)
-            await s.flush()
+        other = await ensure_user(s, "coverage_outsider")
         other_id = other.id
         await s.commit()
     with pytest.raises(ValueError) as err: