Implement round-robin RPC client with cooldown mechanism

- Replace sequential endpoint iteration with round-robin selection
- Add cooldown mechanism to temporarily avoid failed endpoints
- Maintain endpoint health state and timing information
- Add configurable cooldown duration (default 30 seconds)
- Preserve existing API compatibility with new _and_cooldown variants
- Improve resilience by distributing load across healthy endpoints

Co-Authored-By: Ali <[email protected]>

Author: devin-ai-integration[bot]