fix(tools): use correct import path for mistralai SDK

mistralai v2.x is a namespace package — `Mistral` class lives at
`mistralai.client`, not at the top-level `mistralai` module. The
previous `from mistralai import Mistral` raises ImportError at runtime.

Update both production code and test fixture to use the correct path.

tools/transcription_tools.py

@@ -546,7 +546,7 @@ def _transcribe_mistral(file_path: str, model_name: str) -> Dict[str, Any]:
         return {"success": False, "transcript": "", "error": "MISTRAL_API_KEY not set"}
 
     try:
-        from mistralai import Mistral
+        from mistralai.client import Mistral
 
         with Mistral(api_key=api_key) as client:
             with open(file_path, "rb") as audio_file: