test: refactor tests and fix inference edge cases

- Convert protocol and MoE test classes to plain functions
- Add real server/engine integration and generate_async tests
- Isolate test model per test and use pytest tmp_path
- Reset FastAPI engine state after inference tests
- Fix generate_async StopIteration handling on Python 3.12
- Fix HF adapter MoE dense/shared and Gemma qk_norm mapping
- Correct dev dependency httpx2 to httpx

tests/inference/conftest.py

@@ -8,6 +8,13 @@ from fastapi.testclient import TestClient
 from astrai.inference import get_app
 
 
+@pytest.fixture(autouse=True)
+def _cleanup_app_engine():
+    """Reset the lazy FastAPI singleton engine after each inference test."""
+    yield
+    get_app().state.engine = None
+
+
 @pytest.fixture
 def client():
     """Provide a test client for the FastAPI app."""