When a WebSocket server accepts the handshake but immediately closes the
connection (e.g. invalid API key returning close code 1008), the existing
exponential backoff does not help because the handshake keeps succeeding.
This tracks how long each connection survives and emits a non-fatal
ErrorFrame after 3 consecutive sub-5s failures, allowing ServiceSwitcher
failover instead of killing the pipeline.
Fixes#3711
