notify: release held deliveries so shutdown tests fail, not hang (closes #176)

Two shutdown tests hold a delivery inside the test server's handler and
release it from a timer. The deferred timer stop ran before the server
was closed, so a drain that returned early left the handler blocked and
the server's close waited on it until the package timed out. Each test
now defers a release, guarded so the timer and the defer can both call
it, ahead of closing the server. The watchdog comment no longer names a
30-second timeout the test script does not use.

Model: opus-5-5

internal/notify/shutdown_test.go

@@ -143,6 +143,12 @@ func TestDrainWaitsForInFlightDelivery(t *testing.T) {
 	srv := blockingNtfyServer(entered, release, &served)
 	defer srv.Close()
 
+	// srv.Close waits for the handler, so release it however the
+	// test ends; otherwise a drain that returns early hangs the
+	// package instead of failing this test.
+	releaseHandler := sync.OnceFunc(func() { close(release) })
+	defer releaseHandler()
+
 	topicURL, _ := url.Parse(srv.URL)
 
 	svc := notify.NewTestService(http.DefaultTransport)
@@ -167,9 +173,7 @@ func TestDrainWaitsForInFlightDelivery(t *testing.T) {
 	// delay alone.
 	start := time.Now()
 
-	timer := time.AfterFunc(inFlightHold, func() {
-		close(release)
-	})
+	timer := time.AfterFunc(inFlightHold, releaseHandler)
 	defer timer.Stop()
 
 	ctx, cancel := context.WithTimeout(
@@ -268,7 +272,7 @@ func TestDrainBoundedByContextDeadline(t *testing.T) {
 	// all never returns here (the delivery is parked in a backoff
 	// that never fires), so an unbounded drain must fail this
 	// test promptly instead of hanging the package until the test
-	// binary's 30s timeout.
+	// binary's -timeout.
 	returned := make(chan struct{})
 
 	go func() {
@@ -447,6 +451,11 @@ func TestNewRegistersDrainingStopHook(t *testing.T) {
 	srv := blockingNtfyServer(entered, release, &served)
 	defer srv.Close()
 
+	// As in TestDrainWaitsForInFlightDelivery: release the handler
+	// however the test ends, before srv.Close waits for it.
+	releaseHandler := sync.OnceFunc(func() { close(release) })
+	defer releaseHandler()
+
 	lifecycle := &recordingLifecycle{}
 	svc := newNotifyService(t, lifecycle, srv.URL)
 
@@ -472,9 +481,7 @@ func TestNewRegistersDrainingStopHook(t *testing.T) {
 		t.Fatal("delivery never reached the endpoint")
 	}
 
-	timer := time.AfterFunc(inFlightHold, func() {
-		close(release)
-	})
+	timer := time.AfterFunc(inFlightHold, releaseHandler)
 	defer timer.Stop()
 
 	ctx, cancel := context.WithTimeout(