* [client] Wait for signal receive watchdog to stop before reconnect
The per-stream watchReceiveStream goroutine was started fire-and-forget
and never joined. On reconnect a lingering watchdog could still flip
shared client state (receiveStalled, the disconnect notifier) on the
freshly established stream, since cancelStream only cancels its own
stream context.
Track the watchdog with a WaitGroup and wait for it to exit (after
cancelling its stream) before the operation returns, so each reconnect
starts with no stale watchdog.
* [client] Bind signal receive probe to the stream context
The watchdog probe reused the generic Send, which derives its per-attempt
timeouts from the long-lived client context, so cancelStream could not
interrupt an in-flight probe. After joining the watchdog on reconnect,
watchdogWg.Wait() could then block for the full send-attempt chain.
Split Send into a context-aware send and pass the stream context down
through sendReceiveProbe, so cancelStream aborts any in-flight probe and
the watchdog exits promptly.
