[agufagit]

Formats this plugin can't write back, such as WebP, are saved as JPEG, which has no alpha channel, so a transparent image picked on iOS loses its transparency. This saves them as PNG when their metadata reports an alpha channel, using the kCGImagePropertyHasAlpha value from the metadata the plugin already reads. Images without an alpha channel are still saved as JPEG, and scaling already keeps the source's opacity. image_picker_android does the same: ImageResizer saves a bitmap with alpha as PNG.

The ICO and ICNS test images have alpha channels, so their expected extension changes from jpg to png.

As with any PNG, imageQuality doesn't apply to these images: the existing warning is logged and the image keeps its original quality. GIFs keep their own path, so a transparent GIF stays a GIF.

Tested with RunnerTests on an iOS 26.2 simulator: all 119 pass, and 8 checks fail without the change. A test runs a transparent WebP through PHPickerSaveImageToPathOperation, but this wasn't tried by hand through the Photos UI.

Fixes flutter/flutter#195747

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