[image_picker] Add native tests for image, metadata, photo-asset, and PHPicker save utilities (#12536)

Adds native unit tests for previously untested paths in `FLTImagePickerImageUtil`, `FLTImagePickerMetaDataUtil`, `FLTImagePickerPhotoAssetUtil`, and `FLTPHPickerSaveImageToPathOperation` before the Objective-C → Swift migration. Production code is unchanged.

This is a tests-only change, so it does not bump the package version or CHANGELOG.

Native `RunnerTests` go from 72 tests to 88 tests (+16).

### New test cases

**`ImageUtilTests`:**
- `testScaledGIFImage_UsesClampedDelayWhenUnclampedDelayMissing` — GIF scaling uses the clamped delay when unclamped delay is missing

**`MetaDataUtilTests`:**
- `testGetImageMIMETypeFromImageDataUnknownReturnsOther` — unrecognized image bytes map to the Other MIME type
- `testConvertImageIgnoresQualityForPNG` — PNG conversion ignores JPEG quality and stays PNG
- `testConvertImageDefaultsNonJPEGNonPNGToJPEG` — GIF and other non-JPEG/PNG data convert to JPEG

**`PhotoAssetUtilTests`:**
- `testGetAssetFromImagePickerInfoShouldReturnNilIfNotAvailable` — picker info without a PHAsset returns `nil`
- `testGetAssetFromImagePickerInfoShouldReturnAssetIfPresent` — picker info with a PHAsset returns that asset
- `testSaveVideoFromURLReturnsNilWhenSourceIsUnreadable` — missing source file returns `nil`
- `testSaveVideoFromURLCopiesReadableFile` — a readable video is copied to a new path
- `testSaveVideoFromURLReturnsNilWhenCopyFails` — copy failure returns `nil`
- `testSaveImageWithOriginalImageDataNilUsesDefaultJPEG` — `nil` original data saves as `.jpg`
- `testCreateFileReturnsPathWhenWriteFails` — create-file still returns a path when the write fails

**`PickerSaveImageToPathOperationTests`:**
- `testInitWithNilResultReturnsNil` — `nil` PHPicker result does not create an operation
- `testStartWhenCancelledFinishesWithoutSaving` — a cancelled operation finishes without calling the save callback
- `testSaveVideoCopiesFile` — a video item is copied to the saved path
- `testSaveVideoFailsWhenLoadReturnsError` — a load error is reported as `invalid_image`
- `testSaveVideoFailsWhenCopyFails` — a copy error is reported as `flutter_image_picker_copy_video_error`

First part of `image_picker_ios` coverage backfill before the Obj-C → Swift migration for https://github.com/flutter/flutter/issues/119107

## Pre-Review Checklist

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