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/RttCommand.swift

@@ -36,4 +36,9 @@ internal class RttCommand {
     func getRoundTripTime() async throws -> TimeInterval {
         try await promise?.futureResult.get() ?? 0
     }
+    
+    /// Cancel the ping request to avoid promise leaks when connection is closed
+    func cancel() {
+        promise?.fail(NatsError.ClientError.connectionClosed)
+    }
 }