Summary

Support mistralai layouts where Mistral is exposed from mistralai.client instead of the package root.

This keeps Mistral provider initialization working across both import styles and avoids breaking the provider path when the top-level import is unavailable.

Changes

• add a fallback import for Mistral in the Mistral provider client
• add the same fallback in from_provider(..., provider="mistral")
• add a regression test covering the legacy mistralai.client.Mistral import path

Testing

• python -m pytest tests/test_auto_client.py -k "mistral_provider_falls_back_to_legacy_client_module_import or invalid_provider_format or unsupported_provider"
• python -m pytest tests/test_auto_client.py -k "google_provider_runtime_import_error_propagates or generative_ai_provider_runtime_import_error_propagates"

Closes #2137