fix: improve session management and testing infrastructure

- Split get_session into _get_session and get_session to facilitate test mocking
- Add autouse fixture to ensure db_session is properly injected in tests
- Fix generate_waveform method to accept session parameter explicitly

server/tests/conftest.py

@@ -344,6 +344,15 @@ def celery_includes():
     ]
 
 
+@pytest.fixture(autouse=True)
+async def ensure_db_session_in_app(db_session):
+    async def mock_get_session():
+        yield db_session
+
+    with patch("reflector.db._get_session", side_effect=mock_get_session):
+        yield
+
+
 @pytest.fixture
 async def client(db_session):
     from httpx import AsyncClient