fix(test): add fal_client.submit stub to surface matrix test

The plugin switched from fal_client.subscribe() to submit()+handle.get().
The test mock only had subscribe, causing CI failures.

tests/tools/test_video_generation_tool_surface_matrix.py

@@ -46,6 +46,18 @@ def matrix_env(tmp_path, monkeypatch):
         fal_calls.append({"endpoint": endpoint, "arguments": arguments})
         return {"video": {"url": f"https://fake-fal/{endpoint.replace('/','_')}.mp4"}}
     fake_fal.subscribe = _subscribe  # type: ignore
+
+    class _FalHandle:
+        def __init__(self, result):
+            self._result = result
+        def get(self):
+            return self._result
+
+    def _submit(endpoint, arguments=None, headers=None):
+        fal_calls.append({"endpoint": endpoint, "arguments": arguments})
+        return _FalHandle({"video": {"url": f"https://fake-fal/{endpoint.replace('/','_')}.mp4"}})
+    fake_fal.submit = _submit  # type: ignore
+
     monkeypatch.setitem(__import__("sys").modules, "fal_client", fake_fal)
 
     # httpx stub for xAI