[Android] deleteAllCache causes kotlin.Unit IllegalArgumentException and "Reply already submitted"

## Description

On Android, calling `VideoCompress.deleteAllCache()` triggers native MethodChannel errors. The app usually does not crash, but logcat shows repeated errors on every invocation.

## Steps to Reproduce

1. Add `video_compress: ^3.1.4` to a Flutter project
2. Call `await VideoCompress.deleteAllCache()` from Dart (e.g. on screen/cubit init)
3. Run on Android device or emulator
4. Check logcat

## Logs
E/MethodChannel#video_compress( 5117): Failed to handle method call
E/MethodChannel#video_compress( 5117): java.lang.IllegalArgumentException: Unsupported value: 'kotlin.Unit' of type 'class kotlin.Unit'
E/MethodChannel#video_compress( 5117): at io.flutter.plugin.common.StandardMessageCodec.writeValue(StandardMessageCodec.java:297)
E/MethodChannel#video_compress( 5117): at io.flutter.plugin.common.StandardMethodCodec.encodeSuccessEnvelope(StandardMethodCodec.java:61)
E/MethodChannel#video_compress( 5117): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler$1.success(MethodChannel.java:272)
E/MethodChannel#video_compress( 5117): at com.example.video_compress.VideoCompressPlugin.onMethodCall(VideoCompressPlugin.kt:66)
E/MethodChannel#video_compress( 5117): at io.flutter.plugin.common.MethodChannel$IncomingMethodCallHandler.onMessage(MethodChannel.java:267)
E/MethodChannel#video_compress( 5117): at io.flutter.embedding.engine.dart.DartMessenger.invokeHandler(DartMessenger.java:292)
E/MethodChannel#video_compress( 5117): at io.flutter.embedding.engine.dart.DartMessenger.lambda$dispatchMessageToQueue$0$io-flutter-embedding-engine-dart-DartMessenger(DartMessenger.java:319)
E/MethodChannel#video_compress( 5117): at io.flutter.embedding.engine.dart.DartMessenger$$ExternalSyntheticLambda0.run(D8$$SyntheticClass:0)
E/MethodChannel#video_compress( 5117): at android.os.Handler.handleCallback(Handler.java:959)
E/MethodChannel#video_compress( 5117): at android.os.Handler.dispatchMessage(Handler.java:100)
E/MethodChannel#video_compress( 5117): at android.os.Looper.loopOnce(Looper.java:232)
E/MethodChannel#video_compress( 5117): at android.os.Looper.loop(Looper.java:317)
E/MethodChannel#video_compress( 5117): at android.app.ActivityThread.main(ActivityThread.java:8705)
E/MethodChannel#video_compress( 5117): at java.lang.reflect.Method.invoke(Native Method)
E/MethodChannel#video_compress( 5117): at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:580)
E/MethodChannel#video_compress( 5117): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:886)

## Expected Behavior

`deleteAllCache` should complete once and return a boolean to Dart without native errors.

## Actual Behavior

Native side throws:
- `IllegalArgumentException: Unsupported value: 'kotlin.Unit'`
- `IllegalStateException: Reply already submitted`

## Environment

- Package: `video_compress` 3.1.4
- Platform: Android
- Flutter version: 3.35.1

## Root Cause

`deleteAllCache` replies twice to the same MethodChannel call:

**`VideoCompressPlugin.kt` :**
```kotlin
"deleteAllCache" -> {
result.success(Utility(channelName).deleteAllCache(context, result));
}

Utility.kt

fun deleteAllCache(context: Context, result: MethodChannel.Result) {
val dir = context.getExternalFilesDir("video_compress")
result.success(dir?.deleteRecursively())
}

Utility.deleteAllCache already calls result.success(...). The outer result.success(...) in the plugin tries to send kotlin.Unit, which StandardMessageCodec cannot serialize.