Verify that the cost management plugin can be properly configured with API endpoints and credentials.

Related issue: FLPATH-2738

Test Steps:
1. Configure cost management plugin in app-config.yaml
2. Set costManagement.baseUrl to valid endpoint
3. Configure costManagement.apiToken or service account credentials
4. Start the backend and verify configuration is validated on startup
5. Test with invalid credentials and verify appropriate error handling
6. Test with staging vs production targets
7. Verify input sanitization works correctly

Expected Results:

• Plugin accepts valid configuration parameters
• Configuration is validated on startup
• Invalid credentials produce clear error messages
• Separate staging/prod targets can be configured
• Input sanitization prevents injection attacks