[file_selector] Migrate to UIScene

[stuartmorgan-g]

Switches from the global window's view controller to the view controller provided by the new FlutterPluginRegistrar viewController method.

Updates minimum Flutter version to 3.38 for the new API used.

Fixes flutter/flutter#174419

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Footnotes

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[gemini-code-assist]

Code Review

This pull request successfully migrates the file_selector_ios plugin to be compatible with UIScene by using the viewController from FlutterPluginRegistrar instead of relying on the application's key window. The implementation uses dependency injection by introducing a ViewPresenterProvider protocol, which improves testability and modernizes the codebase. The related tests and SDK version constraints have been updated accordingly. The changes are well-implemented, but I have a few minor suggestions to improve code style and documentation consistency.

[stuartmorgan-g]

We could keep this override structure instead of adding ViewPresenterProvider, but I thought it was cleaner to reduce test branching in production code.

[stuartmorgan-g]

This is necessary because FlutterPluginRegistrar is a protocol rather than a class, and the Swift "One Weird Trick" to add protocol conformance only works on classes, not protocols, so we can't do a no-code passthrough to the real registrar using something like line 21 above.

[stuartmorgan-g]

ViewPresenter.swift:42:15: error: value of type 'any FlutterPluginRegistrar' has no member 'viewController'

Ah right, landing any of these changes is going to require publishing a new Flutter pod. @vashworth Do you have the instructions for that, or do I need to try to dig them out?