fix: clear pending pings to avoid promise leaks on connection close/suspend

- Add cancel() method to RttCommand to fail promise on connection close
- Add dequeueAll() method to ConcurrentQueue for batch cleanup
- Call clearPendingPings() in close(), suspend(), and disconnect() methods
- Prevents 'leaking promise' crash when connection is closed while pings are pending

Sources/Nats/ConcurrentQueue.swift

@@ -29,4 +29,13 @@ internal class ConcurrentQueue<T> {
         guard !elements.isEmpty else { return nil }
         return elements.removeFirst()
     }
+    
+    /// Dequeue all elements at once (used for cleanup)
+    func dequeueAll() -> [T] {
+        lock.lock()
+        defer { lock.unlock() }
+        let all = elements
+        elements.removeAll()
+        return all
+    }
 }