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/trainer/test_callbacks.py

@@ -31,13 +31,11 @@ def test_gradient_checkpointing_empty_modules_noop(test_model):
     model = test_model["model"]
     callback = GradientCheckpointingCallback()
 
-    originals = [layer.forward for layer in model.layers]
-
     for layer in model.layers:
         callback._enable(layer)
 
-    for layer, orig in zip(model.layers, originals):
-        assert layer.forward is orig
+    for layer in model.layers:
+        assert not hasattr(layer, "_original_forward")
 
 
 def test_gradient_checkpointing_forward_unchanged(test_model):