[Bugfix] 500 Internal Server Error when tool_choice is incorrect.

[shenoyvvarun]

Summary

This PR fixes the KeyError when tool_choice is incorrectly provided.

Request:
		{
	    "model": "vllm-model",
	    "messages": [ {"role": "user", "content": "what is 5+3"}],
	    "tool_choice": {},
	    "tools": [
	        {
	                "type": "function",
	                "name": "my_caculator",
	                "description": "A simple caculator takes an expression to caculate.",
	                "parameters": {
	                    "type": "object",
	                    "properties": {
	                        "expression": {
	                            "type": "string",
	                            "description": "A expressoin to caculate."
	                        }
	                    },
	                    "required": ["expression"]
	                }
	        }
	    ]
	}
Output:
Internal Server Error(base)

Checklist

• Tested using curl
• After fix:

{
    "object": "error",
    "message": "[{'type': 'value_error', 'loc': ('body',), 'msg': 'Value error, Incorrectly formatted `tool_choice`. Should be like `{\"type\": \"function\", \"function\": {\"name\": \"my_function\"}}`', 'input': {'model': 'vllm-model', 'messages': [{'role': 'user', 'content': 'what is 5+3'}], 'tool_choice': {}, 'tools': [{'type': 'function', 'name': 'my_caculator', 'description': 'A simple caculator takes an expression to caculate.', 'parameters': {'type': 'object', 'properties': {'expression': {'type': 'string', 'description': 'A expressoin to caculate.'}}, 'required': ['expression']}}]}, 'ctx': {'error': ValueError('Incorrectly formatted `tool_choice`. Should be like `{\"type\": \"function\", \"function\": {\"name\": \"my_function\"}}`')}}]",
    "type": "BadRequestError",
    "param": null,
    "code": 400
}

• Lint
• pytest

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀

[DarkLight1337]

Looks good now, thanks for the fix!