fix: local sync task never runs on iOS (#21491)

* fix: local sync task never runs on iOS

* chore: rename ios register method

* refactor from using dart callback to dart entrypoint + more logs

* check if file exists before hashing

* reschedule local sync task

* chore: rename background worker logger

* refactor: move file exists check inside repo

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Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>

mobile/pigeon/background_worker_api.dart

@@ -15,8 +15,7 @@ import 'package:pigeon/pigeon.dart';
 abstract class BackgroundWorkerFgHostApi {
   void enableSyncWorker();
 
-  // Enables the background upload service with the given callback handle
-  void enableUploadWorker(int callbackHandle);
+  void enableUploadWorker();
 
   // Disables the background upload service
   void disableUploadWorker();
@@ -27,6 +26,8 @@ abstract class BackgroundWorkerBgHostApi {
   // Called from the background flutter engine when it has bootstrapped and established the
   // required platform channels to notify the native side to start the background upload
   void onInitialized();
+
+  void close();
 }
 
 @FlutterApi()