## Why
A stalled rendezvous WebSocket upgrade can prevent reconnects, while short-lived connections reset reconnect backoff too early. Blocked relay writes also prevent received pongs from clearing their heartbeat deadline.
## What changed
- Bound rendezvous WebSocket connection attempts with `DEFAULT_REMOTE_EXEC_SERVER_CONNECT_TIMEOUT`.
- Retain exponential reconnect backoff across short-lived connections, resetting it only after a connection lasts at least 120 seconds.
- Process pongs during blocked data writes while preserving the in-flight send and its independent write timeout. Prefer queued pongs over an expired heartbeat deadline.
- Abort the relay writer when its owning connection task is dropped.
## Testing
Add coverage for pongs arriving during blocked writes, missing and queued pongs, independent write timeouts, and writer cancellation. Extend reconnect tests to cover stalled WebSocket upgrades without re-registration and backoff reset after a stable session.
GitOrigin-RevId: 0efa350c2b4ada9b89e23df8909063fbb69f33dc
Alexi Christakis
ddabe594e6