fix: Turn parallel tool calling off for OpenAI (#2737)

letta/llm_api/openai_client.py

@@ -217,6 +217,7 @@ class OpenAIClient(LLMClientBase):
             messages=fill_image_content_in_messages(openai_message_list, messages),
             tools=[OpenAITool(type="function", function=f) for f in tools] if tools else None,
             tool_choice=tool_choice,
+            parallel_tool_calls=False if tools else None,  # Forcibly disable parallel tool calling
             user=str(),
             max_completion_tokens=llm_config.max_tokens,
             # NOTE: the reasoners that don't support temperature require 1.0, not None