feat(server): add fallback model and baseurl in schema (#13375)

fix AI-398

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added support for specifying fallback models for multiple AI
providers, enhancing reliability when primary models are unavailable.
* Providers can now fetch and update their list of available models
dynamically from external APIs.
* Configuration options expanded to allow custom base URLs for certain
providers.

* **Bug Fixes**
* Improved model selection logic to use fallback models if the requested
model is not available online.

* **Chores**
* Updated backend dependencies to include authentication support for
Google services.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/package.json

@@ -86,6 +86,7 @@
     "express": "^5.0.1",
     "fast-xml-parser": "^5.0.0",
     "get-stream": "^9.0.1",
+    "google-auth-library": "^10.2.0",
     "graphql": "^16.9.0",
     "graphql-scalars": "^1.24.0",
     "graphql-upload": "^17.0.0",