test: Check that _convert_chat_completion_chunk_to_streaming_chunk works for MistralChatGenerator

[sjrl]

Related Issues

• partially addresses Check if the util method _convert_chat_completion_chunk_to_streaming_chunk works as expected for integrations that inherit OpenAIChatGenerator haystack#9502

Proposed Changes:

• Add unit test that each Mistral chunk is converted into the expected Haystack StreamingChunk
• Update other tool integration tests to call two tools so the more complicated case is tested
• Added a unit test that explicitly tests that the Mistral Chunks are properly converted into the final Haystack ChatMessage

How did you test it?

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.

[sjrl]

This is done, but putting on hold until this PR deepset-ai/haystack#9525 is merged and the next version of Haystack is released.