fix: add missing tool_calls finish reason

[juburr]

I don't see an example of this anywhere in the docs on platform.openai.com, but "finish_reason": "tool_calls" is used by many OpenAI API providers that I'm trying to integrate with.

The tool calling documentation for all of the providers below support "finish_reason": "tool_calls":

• Meta Llama: https://llama.developer.meta.com/docs/features/tool-calling/
• Qwen: https://qwen.readthedocs.io/en/latest/framework/function_call.html
• LiteLLM: https://docs.litellm.ai/docs/completion/function_call
• LangChain: https://python.langchain.com/docs/how_to/tool_calling/
• Mistral: https://docs.mistral.ai/capabilities/function_calling/
• vLLM: https://github.com/search?q=repo%3Avllm-project%2Fvllm+finish_reason%3D%22tool_calls%22&type=code