feat: set Devstral models as default for Mistral provider

- Changed default_large_model_id from mistral-large to devstral-medium-latest
- Changed default_small_model_id from mistral-small to devstral-small-latest
- This makes Devstral models the default choice when using the Mistral provider

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

Author: ThomsenDrake

internal/providers/configs/mistral.json

@@ -4,8 +4,8 @@
   "api_key": "$MISTRAL_API_KEY",
   "api_endpoint": "https://api.mistral.ai/v1",
   "type": "mistral",
-  "default_large_model_id": "mistral-large",
-  "default_small_model_id": "mistral-small",
+  "default_large_model_id": "devstral-medium-latest",
+  "default_small_model_id": "devstral-small-latest",
   "models": [
     {
       "id": "mistral-large",