feat: enhance authentication handling and API key validation

- Added optional API keys for OpenAI and Cursor to the .env.example file.
- Implemented API key validation in CursorProvider to ensure valid keys are used.
- Introduced rate limiting in Claude and Codex authentication routes to prevent abuse.
- Created secure environment handling for authentication without modifying process.env.
- Improved error handling and logging for authentication processes, enhancing user feedback.

These changes improve the security and reliability of the authentication mechanisms across the application.

libs/types/src/cursor-cli.ts

@@ -217,6 +217,7 @@ export interface CursorAuthStatus {
   authenticated: boolean;
   method: 'login' | 'api_key' | 'none';
   hasCredentialsFile?: boolean;
+  error?: string;
 }
 
 /**