[image_picker] Add native tests for photo-library access and UIImagePicker completion

[victogomez-cs]

Stacked on #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 flutter/flutter#119107

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request adds comprehensive unit tests to ImagePickerPluginTests.m for the iOS image picker plugin, covering permission handling, image/video picking, and scaling scenarios. The review feedback identifies a potential memory corruption or crash issue under ARC in three tests where a block is retrieved from NSInvocation using getArgument:atIndex: into a __strong variable, and suggests declaring these block variables as __unsafe_unretained to prevent over-releasing.

[gemini-code-assist]

Under ARC, retrieving a block from NSInvocation using getArgument:atIndex: directly into a __strong block variable can cause memory corruption or crashes due to over-releasing. Since getArgument:atIndex: does not retain the block, the implicit release by ARC when the __strong variable goes out of scope will result in an over-release. To prevent this, declare the block variable as __unsafe_unretained.

Suggested change

	void (^handler)(PHAuthorizationStatus status);
	[invocation getArgument:&handler atIndex:2];
	__unsafe_unretained void (^handler)(PHAuthorizationStatus status);
	[invocation getArgument:&handler atIndex:2];

[victogomez-cs]

Agreed and fixed. The authorization handler is now retrieved as __unsafe_unretained so ARC doesn’t over-release the NSInvocation argument.

[gemini-code-assist]

Under ARC, retrieving a block from NSInvocation using getArgument:atIndex: directly into a __strong block variable can cause memory corruption or crashes due to over-releasing. Since getArgument:atIndex: does not retain the block, the implicit release by ARC when the __strong variable goes out of scope will result in an over-release. To prevent this, declare the block variable as __unsafe_unretained.

Suggested change

	void (^handler)(PHAuthorizationStatus status);
	[invocation getArgument:&handler atIndex:2];
	__unsafe_unretained void (^handler)(PHAuthorizationStatus status);
	[invocation getArgument:&handler atIndex:2];

[victogomez-cs]

Agreed and fixed. Same fix here: the authorization handler is now __unsafe_unretained.

[gemini-code-assist]

Under ARC, retrieving a block from NSInvocation using getArgument:atIndex: directly into a __strong block variable can cause memory corruption or crashes due to over-releasing. Since getArgument:atIndex: does not retain the block, the implicit release by ARC when the __strong variable goes out of scope will result in an over-release. To prevent this, declare the block variable as __unsafe_unretained.

Suggested change

	void (^handler)(NSData *, NSString *, CGImagePropertyOrientation, NSDictionary *);
	[invocation getArgument:&handler atIndex:4];
	__unsafe_unretained void (^handler)(NSData *, NSString *, CGImagePropertyOrientation, NSDictionary *);
	[invocation getArgument:&handler atIndex:4];

[victogomez-cs]

Agreed and fixed. The PHImageManager result handler is now retrieved as __unsafe_unretained.