Add Mistral transcription provider

[alan747271363-art]

Summary

• add a Mistral Voxtral transcription provider using the audio/transcriptions endpoint
• expose it through --api mistral
• document Mistral environment variables and usage in README/.env.example
• add unit tests for request construction and the optional correction flow

Validation

• py -m compileall src tests
• PYTHONPATH=src py -m sapat.script --help
• PYTHONPATH=src py -m unittest discover -s tests
• py -m build
• git diff --check

This is a companion implementation for daytonaio/content#13.

[alan747271363-art]

CI note: the three failing Continuous AI checks currently report Agent encountered an error from the external Continue agent tasks rather than a project test failure. The PR's local validation was:

• py -m compileall src tests
• PYTHONPATH=src py -m sapat.script --help
• PYTHONPATH=src py -m unittest discover -s tests
• py -m build
• git diff --check

No credentials or external API calls are required for the implementation or tests.