test(anthropic): update mock to async get_final_message

AsyncMessageStream.get_final_message() is async; sync mock caused
TypeError when awaited. Update _MockAnthropicStream to match the
real SDK interface after fix(anthropic) added the await.

Author: xfgong

tests/providers/test_anthropic.py

@@ -337,7 +337,7 @@ async def _iter_events(self):
         for ev in self._events:
             yield ev
 
-    def get_final_message(self):
+    async def get_final_message(self):
         return self._final_message