fix: OAI compat endpoint for meta reference inference provider (#1962)

Test plan:
python tests/verifications/generate_report.py --providers
fireworks,together,llama_meta_ref,openai

Co-authored-by: Eric Huang <erichuang@fb.com>

llama_stack/models/llama/llama4/chat_format.py

@@ -5,6 +5,7 @@
 # the root directory of this source tree.
 
 import io
+import json
 import uuid
 from dataclasses import dataclass
 from typing import Dict, List, Optional, Tuple
@@ -299,6 +300,7 @@ class ChatFormat:
                     call_id=call_id,
                     tool_name=tool_name,
                     arguments=tool_arguments,
+                    arguments_json=json.dumps(tool_arguments),
                 )
             )