test: update test suite for SQLAlchemy 2.0 migration

- Add session fixture for async session management
- Update all test files to use session parameter
- Convert Core-style queries to ORM-style in tests
- Fix controller calls to include session parameter
- Remove obsolete get_database() references

Test progress: 108/195 tests passing

server/tests/test_attendee_parsing_bug.py

@@ -18,6 +18,7 @@ async def test_attendee_parsing_bug():
     """
     # Create a test room
     room = await rooms_controller.add(
+        session,
         name="test-room",
         user_id="test-user",
         zulip_auto_post=False,