Verify that the backend plugin correctly proxies cost management API calls.

Related issue: FLPATH-2737

Test Steps:
1. Deploy RHDH with cost management backend plugin
2. Configure backend plugin with valid credentials
3. Access the cost management frontend
4. Monitor network traffic to verify API calls are proxied
5. Verify responses are returned correctly to the frontend

Expected Results:

• Backend plugin successfully proxies API calls
• Frontend receives correct data from cost management API
• No direct API calls from frontend to external services
• Proxy handles authentication correctly