Fix remaining Swift 6 concurrency issues

[kieran-osgood-shopify]

Stack Context

This stack incrementally prepares the Swift SDK and sample apps for Swift 6, with the final PR enabling Swift 6 language mode.

What?

Adds the last compatibility fixes needed immediately before flipping the language mode:

• marks remaining shared state / mock types safe for Swift 6 checking
• keeps the Checkout Kit configuration API usable from existing call sites
• updates SwiftUI and web view tests for main-actor-isolated APIs
• removes obsolete cache invalidation test setup from the renamed checkout web view flow

Why?

Keeping these fixes in a separate PR keeps the final language-mode PR focused on the actual compiler setting changes. This preserves the incremental stack so each step can be reviewed independently before kieran-osgood/05-13-featswift6languagemode turns Swift 6 on.

How to test

shadowenv exec --dir /Users/ko/src/github.com/Shopify/checkout-kit -- /opt/dev/bin/dev swift test

[kieran-osgood-shopify]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Fix remaining Swift 6 concurrency issues #273 👈 (View in Graphite)
• Fix Swift 6 sample concurrency issues #99 : 1 other dependent PR (#92 )
• Fix Swift 6 test target isolation #91 : 1 other dependent PR (#98 )
• Align PassKit protocol abstractions with Swift 6 #90
• Isolate wallet controllers to the main actor #89
• #88 Continuation: Make AcceleratedCheckouts.Configuration Sendable #274
• AC Storefront: GraphQL values Sendable #88
• Make generated JSON coding key Sendable #87
• Logger: NSLock guaranteed #86
• ShopifyCheckoutKit.Configuration: NSLock guaranteed #85
• UI: @MainActor isolated #84
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[github-actions]

React Native — Coverage Report

Lines	Statements	Branches	Functions
	91.66% (319/348)	87.86% (181/206)	100% (82/82)

[kieran-osgood-shopify]

Closing as obsolete: this final cleanup branch has been dissolved into the semantic Swift 6 branches (#85, #89, #90, and #91), with generated/codegen cleanup in #87.