[image_picker_ios] Return an error for undecodable image data

[TeddyYeung]

PHPicker can provide data for a type that conforms to UTTypeImage even when UIImage cannot decode the bytes. The linked issue reports this with an AVIF selection. The existing path continues with a nil image and can treat the selection as successfully saved.

This change completes the operation with the existing invalid_image error when decoding fails. It also adds a regression test that verifies the callback returns no path and that the NSOperation finishes.

Fixes flutter/flutter#132154

No public API documentation changes are needed because this uses the existing image-picker error contract.

Tests run:

• flutter_plugin_tools format --packages image_picker_ios --fail-on-change
• flutter_plugin_tools analyze --ios --packages image_picker_ios
• flutter_plugin_tools dart-test --packages image_picker_ios
• flutter_plugin_tools native-test --ios --no-integration --packages image_picker_ios
• flutter_plugin_tools validate --packages image_picker_ios
• flutter_plugin_tools license-check
• (from packages/image_picker/image_picker_ios) dart pub publish --dry-run

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Code Review

This pull request updates the iOS implementation of the image_picker plugin to handle cases where selected image data cannot be decoded. In FLTPHPickerSaveImageToPathOperation.m, a check has been added to verify if the UIImage is nil after initialization; if so, the operation completes with an 'invalid_image' FlutterError. A corresponding unit test has been added to PickerSaveImageToPathOperationTests.m to verify this behavior, and the package version has been bumped to 0.8.13+7 in both pubspec.yaml and CHANGELOG.md. There are no review comments to address.