feat: add fileSize property to AssetEntity

[dev-ale]

Summary

Adds a fileSize property to AssetEntity that reads the file size in bytes from platform metadata at query time, without downloading files from iCloud or other cloud storage.

Motivation

Currently, the only way to get an asset's file size is via asset.file or asset.originFile, which triggers full file downloads on iOS (including from iCloud). This is prohibitively slow for apps that need to display or calculate file sizes for many assets — for example, photo declutter apps showing "freed space" estimates.

Changes

Layer	File	Change
Dart	lib/src/types/entity.dart	Add fileSize field (nullable int?) to AssetEntity constructor, field declaration, and copyWith
Dart	lib/src/utils/convert_utils.dart	Read data['fileSize'] in convertMapToAsset()
iOS/macOS	darwin/.../PMAssetPathEntity.h	Add fileSize property to PMAssetEntity
iOS/macOS	darwin/.../PMManager.m	Populate fileSize from PHAssetResource metadata via value(forKey: "fileSize")
iOS/macOS	darwin/.../PMConvertUtils.m	Serialize fileSize in both convertPMAssetToMap: and convertPHAssetToMap:
Android	core/entity/AssetEntity.kt	Add fileSize: Long field
Android	core/utils/IDBUtils.kt	Add MediaStore.MediaColumns.SIZE to storeImageKeys and storeVideoKeys
Android	extension/CursorExtensions.kt	Read SIZE column from cursor, pass to entity
Android	core/utils/ConvertUtils.kt	Serialize fileSize to map
OHOS	PhotoAssetHandler.ets	Read SIZE (already in fetchColumns) and add ['fileSize', file_size] to map
Tests	test/convert_utils_test.dart	New test file: ConvertUtils.convertMapToAsset fileSize parsing + AssetEntity.copyWith preservation
Docs	CHANGELOG.md	Add entry under Unreleased

iOS implementation note

On iOS/macOS, PHAssetResource.value(forKey: "fileSize") is used to read file size from the Photos database metadata. This is a well-known KVC technique that reads locally cached metadata — it does not trigger iCloud downloads. While the fileSize key is not part of Apple's documented public API for PHAssetResource, it has been stable since the Photos framework was introduced and is widely used in the iOS ecosystem.

The implementation selects the primary resource (.photo, .video, .fullSizePhoto, or .fullSizeVideo) to report the most relevant file size, falling back to the first available resource.

Usage

final AssetEntity asset = ...;

// File size in bytes — available immediately, no download needed
final int? bytes = asset.fileSize;
if (bytes != null) {
  print('File size: ${(bytes / 1024 / 1024).toStringAsFixed(1)} MB');
}

Testing

• dart analyze lib/ test/ — no issues
• flutter test — all 15 tests pass (6 new tests added)
• Manual testing on iOS device with iCloud photos
• Manual testing on Android device

Closes #1369
Addresses #1366

[AlexV525]

Follow how other fields are being imported.

[AlexV525]

Seems to be duplicate long

[AlexV525]

Could we clarify which file (compare to .file or .originalFile) the size indicated here refers to?

[AlexV525]

Did you get any doc reference on this key?

[hitendigi]

I have tested this on both Android and iOS device and its working as expected, before this I had to run a separate process to fetch file size after getting image ids from the library. Using the same method that this PR used. Request to merge in stable release.