Refactor StackClientInterface for improved URL fallback handling

- Updated the fallback logic in StackClientInterface to introduce a sticky mode, allowing for more efficient URL management during network failures.
- Enhanced test cases to validate the new fallback behavior, ensuring proper handling of primary and fallback URLs.
- Simplified the retry mechanism and improved the structure of the fallback logic for better maintainability.

These changes enhance the robustness of the client interface by optimizing how it handles URL requests and failures.

Author: mantrakp04