Enhance error handling for Mistral API responses

[jka236]

Description

Previously, MistralGenerator._call_model treated all SDKError exceptions the same way — wrapping them in GeneratorBackoffTrigger and retrying indefinitely. This meant auth failures (401/403) would silently retry forever with no actionable feedback, and rate limit errors (429) were not distinguished from other transient failures.

This PR:

• Raises BadGeneratorException with a clear message for 401/403 auth failures so runs fail fast instead of retrying
• Raises RateLimitHit for 429 responses and adds it to the backoff exception list so rate limits trigger the existing backoff/retry logic
• Falls through to GeneratorBackoffTrigger for all other SDKError codes (preserving existing retry behavior for 5xx etc.)
• Fixes test_mistral_chat which was passing a raw Message to generate() instead of the required Conversation object

#1356

Verification

• Set MISTRAL_API_KEY to a valid key and run garak --model_type mistral --model_name mistral-small --probes dan.Ablation_Dan_11_0
• Set MISTRAL_API_KEY to an invalid key — confirm the run fails immediately with BadGeneratorException and a clear auth error message, rather than retrying
• Run the tests: python -m pytest tests/generators/test_mistral.py -v
• Confirm test_mistral_403_raises_bad_generator_exception and test_mistral_429_raises_rate_limit_hit both pass without a real API key