[image_picker] Update for UIScene compatibility

[stuartmorgan-g]

Replaces the code that used the key window's root view controller with a call to the new registrar viewController method to get the actual Flutter content's view controller.

Introduces the same protocol abstraction now used in a number of our plugins, so that a stub can be injected without having to mock (which would be a barrier to Swift migration) or fake (which is fragile since it would have to be complete, and methods can be added over time) the entire Flutter plugin registrar.

Fixes flutter/flutter#174418

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• 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 updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption 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.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[gemini-code-assist]

Code Review

This pull request refactors the image_picker_ios plugin to improve UIScene compatibility by abstracting view controller access behind a FIPViewProvider protocol. This is a solid approach that also improves testability. The changes are mostly correct, but I've found a critical issue where many tests were not updated to use the new initializer, potentially masking issues and reducing test effectiveness. I've also included a couple of minor suggestions for improving code style and robustness.

[gemini-code-assist]

The introduction of this new initializer is a great step for testability. However, the default init is still available and used in many tests in ImagePickerPluginTests.m (e.g., testPluginPickImageDeviceBack, testPluginPickVideoDeviceFront, etc.). When [[FLTImagePickerPlugin alloc] init] is called, the viewProvider property remains nil. Since it's declared nonnull, this is incorrect and can lead to unexpected behavior or crashes. Currently, it seems to result in view controllers not being presented during tests because messages to the nil viewProvider are silently ignored. This makes those tests less effective.

To fix this, you should:

1. Make the default init unavailable by adding - (instancetype)init NS_UNAVAILABLE; to the FLTImagePickerPlugin's primary interface in FLTImagePickerPlugin.h.
2. Update all call sites of [[FLTImagePickerPlugin alloc] init] in the tests to use the new initWithViewProvider:, providing a StubViewProvider as is done in testPickVideoSetsCurrentRepresentationMode. This will ensure all tests correctly provide a view controller for presentation.

[stuartmorgan-g]

Currently, it seems to result in view controllers not being presented during tests because messages to the nil viewProvider are silently ignored. This makes those tests less effective.

This is what the tests will do anyway since the view controller will be null, which is why I didn't bother changing all the tests originally, but the point about violating the non-nullability annotation is fair. I added a bit of boilerplate to all of the tests so that won't need to be done as part of a Swift migration.