fix: tool call encoded twice (#2034)

# What does this PR do?


## Test Plan
LLAMA_STACK_CONFIG=http://localhost:5002 pytest -s -v
tests/integration/inference --safety-shield meta-llama/Llama-Guard-3-8B
--vision-model meta-llama/Llama-4-Scout-17B-16E-Instruct --text-model
meta-llama/Llama-4-Scout-17B-16E-Instruct

llama_stack/models/llama/llama4/chat_format.py

@@ -303,6 +303,7 @@ class ChatFormat:
                     arguments_json=json.dumps(tool_arguments),
                 )
             )
+            content = ""
 
         return RawMessage(
             role="assistant",