`launchHeartbeat` could miss a stale-threshold update during startup if
`SetStaleAfter` ran after the heartbeat ticker was created but before
the goroutine subscribed to `thresholdChan`. In that case, the heartbeat
kept the old interval until a future tick, and the mock-clock test could
time out waiting for `Ticker.Reset` without advancing time.
Subscribe to `thresholdChan` before reading the heartbeat interval so
the channel consistently invalidates the interval. The regression test
now changes the threshold while ticker creation is trapped, making the
startup race deterministic.
Closes https://github.com/coder/internal/issues/1513
Ethan