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_trash.py

@@ -2,14 +2,7 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
-
-
-def _make_mock_session():
-    s = AsyncMock()
-    s.__aenter__ = AsyncMock(return_value=s)
-    s.__aexit__ = AsyncMock(return_value=False)
-    s.commit = AsyncMock()
-    return s
+from tests.helpers import make_mock_session
 
 
 def _exists_result(found=True):
@@ -20,7 +13,7 @@ def _exists_result(found=True):
 
 @pytest.mark.asyncio
 async def test_delete_note_returns_batch_and_commits():
-    session = _make_mock_session()
+    session = make_mock_session()
     # exists-check, then the subtree descent: one child-lookup (no children
     # here) + one _set stamping the whole subtree.
     no_children = MagicMock()
@@ -38,7 +31,7 @@ async def test_delete_note_returns_batch_and_commits():
 
 @pytest.mark.asyncio
 async def test_delete_returns_none_when_not_found():
-    session = _make_mock_session()
+    session = make_mock_session()
     session.execute = AsyncMock(return_value=_exists_result(False))
     with patch("scribe.services.trash.async_session") as cls:
         cls.return_value = session
@@ -52,7 +45,7 @@ async def test_delete_returns_none_when_not_found():
 
 @pytest.mark.asyncio
 async def test_delete_project_cascades_to_notes_milestones_project_rules_and_suppressions():
-    session = _make_mock_session()
+    session = make_mock_session()
     # exists-check + 6 cascade ops:
     #   notes (soft) → milestones (soft) → project-scoped rules (soft) →
     #   project_rule_suppressions (hard DELETE) → project_topic_suppressions (hard DELETE) →
@@ -73,7 +66,7 @@ async def test_delete_project_cascades_to_notes_milestones_project_rules_and_sup
 
 @pytest.mark.asyncio
 async def test_delete_rulebook_cascades_topics_and_rules():
-    session = _make_mock_session()
+    session = make_mock_session()
     topic_ids_result = MagicMock()
     topic_ids_result.scalars.return_value.all.return_value = [10, 11]
     # exists-check, topic-id select, then 3 updates (rules, topics, rulebook)
@@ -96,7 +89,7 @@ def _rowcount_result(n):
 
 @pytest.mark.asyncio
 async def test_restore_clears_batch_across_all_models():
-    session = _make_mock_session()
+    session = make_mock_session()
     # 6 soft-deletable models, each returns a rowcount
     session.execute = AsyncMock(side_effect=[_rowcount_result(i) for i in [2, 0, 1, 1, 0, 0]])
     with patch("scribe.services.trash.async_session") as cls:
@@ -110,7 +103,7 @@ async def test_restore_clears_batch_across_all_models():
 
 @pytest.mark.asyncio
 async def test_purge_expired_skips_when_retention_zero():
-    session = _make_mock_session()
+    session = make_mock_session()
     session.execute = AsyncMock()
     with patch("scribe.services.trash.async_session") as cls:
         cls.return_value = session
@@ -122,7 +115,7 @@ async def test_purge_expired_skips_when_retention_zero():
 
 @pytest.mark.asyncio
 async def test_purge_expired_deletes_across_models_when_positive():
-    session = _make_mock_session()
+    session = make_mock_session()
     session.execute = AsyncMock(side_effect=[_rowcount_result(1) for _ in range(6)])
     with patch("scribe.services.trash.async_session") as cls:
         cls.return_value = session
@@ -157,7 +150,7 @@ def test_owner_clause_scopes_every_model():
 
 @pytest.mark.asyncio
 async def test_list_trash_groups_by_batch():
-    session = _make_mock_session()
+    session = make_mock_session()
 
     def _note(id, batch, title):
         from datetime import datetime, timezone