Refactor DownloadManager image loading and caching

[porter-stripe]

Summary

Refactors DownloadManager around a Swift async image-loading API with separate decoded-image and URL response caches. Payment method image consumers now use an immediately available cached image for initial rendering and an async load when the remote image is needed, with cancellation on cell reuse. The follow-up preserves synchronous promotion of URLCache images and the existing 1x1 transparent fallback behavior.

Motivation

Simplify image loading and caching

Testing

• New and updated DownloadManager, card-art, and payment-method image tests
• ci_scripts/run_tests.rb --test StripePaymentSheetTests/DownloadManagerTest --test StripePaymentSheetTests/STPPaymentMethodCardArtImageTest --test StripePaymentSheetTests/PaymentSheetPaymentMethodTypeTest
• Formatting and lint checks

Changelog

N/A

[github-actions]

⚠️ Public API changes detected:

StripePaymentSheet

Public API

-import CoreGraphics

If you are adding a new public API consider the following:

• Do these APIs need to be public or can they be protected with @_spi(STP)?
• If these APIs need to be public, assess whether they require an API review.

If you are modifying or removing a public API:

• Does this require a breaking version change?
• Do these changes require API review?

If you confirm these APIs need to be added/updated and have undergone necessary review, add the label modifies public API to this PR to acknowledge the interface change.
Additionally, if you modified or removed an existing API, ensure you update the changelog to reflect the necessary version bump for your changes. Regular public API changes require a MAJOR version bump, and SPI API changes (other than declarations using only @_spi(STP) and/or @_spi(ReactNativeSDK)) require a MINOR or MAJOR version bump.

ℹ️ If this comment appears to be left in error, make sure your branch is up-to-date with master.

[github-actions]

⚠️ New dead code detected in this PR but ignored:

DownloadManager.swift:92 warning: Unused function 'clearCache()'

This dead code check has been bypassed with the skip dead code check label.

ℹ️ If this comment appears to be left in error, double check that the flagged code is actually used and/or make sure your branch is up-to-date with master.

[find-dead-code]

[porter-stripe]

seems previously broken