In RequestCoalescer.Do, the primary path calls fn() before storing the
response, deleting the inflight key and calling wg.Done(). If fn() panics,
all three steps are skipped: the sync.Map entry leaks and every coalesced
waiter remains blocked on wg.Wait() forever (permanent deadlock for that
key). Route release through a defer so waiters are always released and the
entry cleared, even on panic.
Test: TestRequestCoalescer_PanicSafety (a panicking primary must not poison
the key for subsequent calls).
lukaszraczylo
