⬆️(backend) upgrade pydantic-ai-slim and remove mistralai

We upgraded pydantic-ai-slim to the latest version and add mistral has
extra dependency. Doing this, the direct dependance to mistral can be
removed and will be managed with pidantic-ai-slim package. The import of
mistralai has changed, we have to updagre them from mistralai to
mistralai.client

src/backend/core/services/ai_services/legacy.py

@@ -9,7 +9,7 @@ from django.core.exceptions import ImproperlyConfigured
 
 from langfuse import get_client, observe
 from langfuse.openai import OpenAI as OpenAI_Langfuse
-from mistralai import Mistral
+from mistralai.client import Mistral
 
 from core import enums