fix(mistral): forward penalties in completions

[he-yufeng]

Summary

• forward presence_penalty and frequency_penalty in non-streaming Mistral completion requests
• keep the non-streaming path aligned with the existing streaming Mistral request path
• record the outgoing Mistral mock kwargs in the existing model tests and add a regression test

To verify

• python -m py_compile pydantic_ai_slim\pydantic_ai\models\mistral.py tests\models\test_mistral.py
• $env:PYTHONUTF8='1'; uv run ruff check pydantic_ai_slim\pydantic_ai\models\mistral.py tests\models\test_mistral.py
• $env:PYTHONUTF8='1'; uv run pytest tests\models\test_mistral.py::test_non_streaming_forwards_penalties -q --basetemp .tmp\pytest-5657 -p no:cacheprovider

Related: #5657