Enhancement: Swift Concurrency Support for PaymentMethodMessagingView

[esreli]

See relevant issue: #4447

Summary

This PR refactors the initialization flow of PaymentMethodMessagingView to use Swift concurrency (async/await) rather than relying on a completion callback. It also introduces cancellation checks to ensure that any ongoing tasks are correctly canceled before initiating new work (such as when trait collection changes in rapid succession).

Motivation

• Async/Await Simplification: By switching from completion closures to async/await, the code becomes more readable and makes better use of Swift concurrency’s structured approach.
• Cancellation Safety: In traitCollectionDidChange, quickly toggling between appearance modes (e.g., Light/Dark) can trigger multiple UI updates and server fetches in quick succession. Introducing a shared Task reference that is canceled before starting a new one ensures we don’t perform redundant or conflicting work.
• Maintain UI Accuracy: Canceling outdated tasks prevents older (stale) data from overwriting the latest UI changes.

Testing

Happy to test. I did not see any relevant tests in the sibling test module.

Changelog

• [Changed] PaymentMethodMessagingView.create(configuration:) to use an async initializer.
• [Changed] Updated traitCollectionDidChange(_:) to cancel and restart any in-flight tasks, ensuring stale work isn’t applied after a trait change.

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