fix: sanity check on call state before CXEndCall transaction*

Calling
`callKitCallController.request(CXEndCallAction)` with no matching call to UUID will result in the following error:
> End Call Failed: The operation couldn’t be completed.
> com.apple.CallKit.error.requesttransaction error 4

Using the CXCallObserverDelegate call observer, we can check if the call is present before requesting the transaction.

Author: cybex-dev

ios/Classes/SwiftTwilioVoicePlugin.swift

@@ -553,6 +553,11 @@ public class SwiftTwilioVoicePlugin: NSObject, FlutterPlugin,  FlutterStreamHand
         }
     }
 
+    // Check if a call with a given UUID exists
+    func isCallActive(uuid: UUID) -> Bool {
+        return activeCalls[uuid] != nil
+    }
+    
     func incomingPushHandled() {
         if let completion = self.incomingPushCompletionCallback {
             self.incomingPushCompletionCallback = nil
@@ -905,6 +910,13 @@ public class SwiftTwilioVoicePlugin: NSObject, FlutterPlugin,  FlutterStreamHand
 
         self.sendPhoneCallEvents(description: "LOG|performEndCallAction method invoked", isError: false)
 
+        // check if call is still active, preventing a race condition ending the call throwing an End Call Failed transaction error 4 error
+        guard isCallActive(uuid: uuid) else {
+            print("Call not found or already ended. Skipping end request.")
+            self.sendPhoneCallEvents(description: "Call Ended", isError: false)
+            return
+        }
+        
         let endCallAction = CXEndCallAction(call: uuid)
         let transaction = CXTransaction(action: endCallAction)