Add dummy tool call id for models that doesn't return it

[AlexEnrique]

Attempt to fix #895

Changes

• Added dummy_tool_call_id to _utils.py
• Used dummy_tool_call_id in models/gemini.py and models/mistral.py, where it was found that tool call id could be None:
• Updates test for Gemini after using dummy_tool_call_id. No tests were added since the updated tests already verify the usage of the dummy id.
• Added test_request_tool_call_with_no_id_from_model test for Mistral to test usage of dummy_tool_call_id when MistralToolCall has id == None.

[Kludex]

I think the problem is that we are not sending a tool_call_id to Gemini, isn't it?

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EDIT: I see the field id from Gemini, but I don't understand when it's returned.

[Kludex]

I've merged a fix for the issue on #1229

Thanks for the PR! :)

[AlexEnrique]

Sorry, I couldn't work on this since the PR.

Thanks for the fix and for the great library you from Pydantic are building!