⬆️(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/tests/test_services_ai_services.py

@@ -10,7 +10,7 @@ from django.core.exceptions import ImproperlyConfigured
 from django.test.utils import override_settings
 
 import pytest
-from mistralai import Mistral
+from mistralai.client import Mistral
 from openai import OpenAI, OpenAIError
 from pydantic_ai.models.mistral import MistralModel
 from pydantic_ai.models.openai import OpenAIChatModel