[image_picker] Add native tests for photo-library access and UIImagePicker completion (#12540)

Stacked on https://github.com/flutter/packages/pull/12539.

Adds native unit tests for previously untested paths in `FLTImagePickerPlugin.m` (photo-library authorization, `UIImagePicker` launch, and `didFinishPickingMediaWithInfo:`) 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.

`ImagePickerPluginTests` go from 52 tests to 70 tests (+18).

### New test cases

**Photo-library access:**
- `testLaunchUIImagePickerGalleryWithoutFullMetadataSkipsAuthorization` — gallery without full metadata skips the authorization check
- `testPhotoAccessDeniedReturnsError` — denied photo access returns an error
- `testPhotoAccessRestrictedReturnsError` — restricted photo access returns an error
- `testPhotoAccessAuthorizedShowsLibrary` — authorized access presents the library
- `testPhotoAccessNotDeterminedGrantedShowsLibrary` — prompt granted presents the library
- `testPhotoAccessNotDeterminedDeniedReturnsError` — prompt denied returns an error
- `testPhotoAccessLimitedReturnsDeniedError` — Limited access falls through to denied
- `testPhotoAccessUnknownStatusTreatedAsDenied` — unknown status is treated as denied

**`launchUIImagePickerWithSource:`:**
- `testLaunchUIImagePickerInvalidSourceReturnsError` — an invalid source returns an error
- `testLaunchUIImagePickerSetsImageAndVideoMediaTypes` — image/video media types and duration are set on the picker

**`imagePickerController:didFinishPickingMediaWithInfo:`:**
- `testImagePickerDidFinishPickingOriginalImage` — original image is saved
- `testImagePickerDidFinishPickingPrefersEditedImage` — edited image is preferred over original
- `testImagePickerDidFinishPickingScalesImage` — the image is scaled when requested
- `testImagePickerDidFinishPickingWithFullMetadataAsset` — full-metadata path uses `PHImageManager` when an asset is present
- `testImagePickerDidFinishPickingFullMetadataWithoutAsset` — full metadata without an asset still completes
- `testImagePickerDidFinishPickingVideo` — video is copied successfully
- `testImagePickerDidFinishPickingVideoCopyFailure` — a video copy failure is reported
- `testImagePickerDidFinishPickingIgnoredWhenNoCallContext` — no `callContext` is a no-op

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

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