test: add test for CancelationPubSub error path

Add TestCancelationPubSubReceiveError to verify that when
Receive() fails in CancelationPubSub(), an error is returned
and the pubsub connection is not leaked.

This provides test coverage for the pubsub.Close() fix that
was missing in the previous commit.

Bahtya

internal/rdb/rdb_test.go

@@ -3274,6 +3274,29 @@ func TestCancelationPubSub(t *testing.T) {
 	mu.Unlock()
 }
 
+func TestCancelationPubSubReceiveError(t *testing.T) {
+	// Use a client connected to a non-existent Redis server to trigger
+	// a Receive() error. This verifies that the pubsub connection is
+	// closed on error, preventing connection leaks.
+	client := redis.NewClient(&redis.Options{
+		Addr: "localhost:0", // invalid port — connection will fail
+	})
+	r := NewRDB(client)
+	defer r.Close()
+
+	pubsub, err := r.CancelationPubSub()
+	if err == nil {
+		// If no error, we must clean up the pubsub.
+		if pubsub != nil {
+			pubsub.Close()
+		}
+		t.Fatal("(*RDB).CancelationPubSub() expected to return an error when redis is unreachable")
+	}
+	if pubsub != nil {
+		t.Error("(*RDB).CancelationPubSub() expected nil pubsub on error")
+	}
+}
+
 func TestWriteResult(t *testing.T) {
 	r := setup(t)
 	defer r.Close()