Add explicit validation error for tool calls.

[juliendenize]

Purpose

Add explicit validation error for tool calls.

Fix #34225

Test Plan

Checked with these messages in the request:

    {
        "role": "user",
        "content": [
            {
                "type": "text",
                "text": "Hey call a tool",
            },
        ],
    },
    {
      "role":"assistant",
      "content":"...",
      "tool_calls":"should_be_serialized_object_but_its_str"
    },
    {
        "role": "user",
        "content": [
            {
                "type": "text",
                "text": "What is the weather in Paris?",
            },
        ],
    },
]

Test Result

Raised correctly:

(APIServer pid=106559) ERROR 02-12 14:55:36 [serving.py:310]   File "/mnt/vast/home/julien.denize/workspace/vllm/vllm/tokenizers/mistral.py", line 77, in maybe_serialize_tool_calls
(APIServer pid=106559) ERROR 02-12 14:55:36 [serving.py:310]     raise ValueError(
(APIServer pid=106559) ERROR 02-12 14:55:36 [serving.py:310]     ...<2 lines>...
(APIServer pid=106559) ERROR 02-12 14:55:36 [serving.py:310]     ) from e
(APIServer pid=106559) ERROR 02-12 14:55:36 [serving.py:310] ValueError: Validating messages' `tool_calls` raised an error. Please ensure `tool_calls` are iterable of tool calls.

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[gemini-code-assist]

Code Review

This pull request introduces an explicit validation error for tool_calls to improve error handling when invalid data is provided. The change correctly catches pydantic.ValidationError and raises a more user-friendly ValueError. I've suggested a refactoring to make the implementation more concise and Pythonic by using list() to consume the iterator, which simplifies the code while maintaining the same logic.