[FEATURE REQ] [Thin Client][Gateway V2] HTTP/2 channel health checker — PING-based liveness and broken connection eviction

## Summary

HTTP/2 connections to the thin client proxy can become stale/broken without the SDK knowing. PING-based health checking detects and evicts dead channels; GOAWAY handling enables graceful rotation.

## Acceptance Criteria

- [ ] PR created and merged
- [ ] PING frames detect dead HTTP/2 connections within health check interval
- [ ] Dead channels evicted from connection pool
- [ ] GOAWAY frames trigger graceful connection rotation
- [ ] Healthy connections preserved across health check cycles
- [ ] All 3 netem tests pass