fix(coalesce): release waiters when primary fn panics

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).

request_coalescing.go

@@ -138,6 +138,15 @@ func (rc *RequestCoalescer) Do(key string, fn func() (*CoalescedResponse, error)
 	rc.inflightCount.Add(1)
 	defer rc.inflightCount.Add(-1)
 
+	// Always release waiters and clear the map entry, even if fn() panics.
+	// Without this, a panicking fn() leaves the key in the inflight map and
+	// the WaitGroup counter positive, so every coalesced waiter for that key
+	// blocks on wg.Wait() forever (permanent deadlock + leak).
+	defer func() {
+		rc.inflight.Delete(key)
+		inflight.wg.Done()
+	}()
+
 	// Execute the request
 	response, err := fn()
 
@@ -152,10 +161,6 @@ func (rc *RequestCoalescer) Do(key string, fn func() (*CoalescedResponse, error)
 	}
 	inflight.mu.Unlock()
 
-	// Clean up and notify waiters
-	rc.inflight.Delete(key)
-	inflight.wg.Done()
-
 	// Log statistics
 	waiters := inflight.waiters.Load()
 	duration := time.Since(inflight.createdAt)