✨(backend) allow to use new ai feature using mistral sdk

We give the possibility, for the new ai feature, to choose between using
the OpenAI or Mistral sdk. For instances having access to the mistral
infrastructure, using it is mor appropriated than using the openai
compatible chat model.

src/backend/core/tests/external_api/test_external_api_documents_ai.py

@@ -14,6 +14,7 @@ import pytest
 from rest_framework.test import APIClient
 
 from core import factories, models
+from core.services.ai_services import configure_legacy_openai_client
 from core.tests.documents.test_api_documents_ai_proxy import (  # pylint: disable=unused-import
     ai_settings,
 )
@@ -23,6 +24,13 @@ pytestmark = pytest.mark.django_db
 # pylint: disable=unused-argument
 
 
+@pytest.fixture(autouse=True)
+def clear_openai_client_config():
+    """Clear the configure_legacy_openai_client cache."""
+    yield
+    configure_legacy_openai_client.cache_clear()
+
+
 def test_external_api_documents_ai_transform_not_allowed(
     user_token, resource_server_backend, user_specific_sub
 ):