⬆️(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/pyproject.toml

@@ -53,7 +53,6 @@ dependencies = [
     "langfuse==3.11.2",
     "lxml==6.1.1",
     "markdown==3.10.2",
-    "mistralai==2.7.2",
     "mozilla-django-oidc==5.0.2",
     "nested-multipart-parser==1.6.0",
     "openai==2.48.0",
@@ -61,7 +60,7 @@ dependencies = [
     "psycopg[binary,pool]==3.3.4",
     "pycrdt==0.14.1",
     "pydantic==2.13.4",
-    "pydantic-ai-slim[openai,logfire,web]==1.58.0",
+    "pydantic-ai-slim[openai,mistral,logfire,web]==1.107.1",
     "PyJWT==2.13.0",
     "python-magic==0.4.27",
     "redis<6.0.0",