Migrate Swift sample from WKWebView to ASWebAuthenticationSession

[markmur]

What changes are you making?

Migrate the CheckoutKitSwiftDemo customer account sign-in and sign-out flows from an app-owned WKWebView to ASWebAuthenticationSession.

This gives OAuth ownership to the system authentication UI and uses the shared browser session by default, allowing the demo to reuse existing Shopify browser state and authentication methods such as passkeys. The authentication session now receives its callback directly, so the customer account URL handling in SceneDelegate and the embedded login sheet are no longer needed.

The updated flow also:

• accepts an explicit presentation anchor and exposes async sign-in/sign-out operations
• validates the callback URL and OAuth state before exchanging the authorization code
• distinguishes cancellation, invalid callbacks, and authorization failures
• prevents multiple authentication sessions from running concurrently
• performs interactive browser logout while always clearing the local session
• resets the cart and invalidates Checkout Kit when authentication state changes
• avoids logging token claims or customer information
• form-encodes token requests correctly

This PR keeps the implementation scoped to the demo app. Moving the customer account functionality into a reusable SDK product is handled separately.

How to test

1. Configure the Swift demo with a Customer Account API shop ID, client ID, and registered redirect URI.
2. Build and launch CheckoutKitSwiftDemo.
3. Tap Log in and then Sign In.
4. Continue through the system authentication prompt and verify the account screen shows the authenticated state.
5. Open checkout and verify the cart uses the authenticated customer account.
6. Cancel a sign-in attempt and verify the demo returns without presenting an error.
7. Tap Sign Out, complete the browser logout, and verify the local account and cart are cleared.
8. Start sign-in again and verify the previous Shopify browser session has been logged out.

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Before you merge

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines
• I have read and agree with the Code of Conduct
• I've updated the relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)

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Releasing a new Swift version?

• I have bumped the version in ShopifyCheckoutKit.podspec
• I have bumped the version in platforms/swift/Sources/ShopifyCheckoutKit/ShopifyCheckoutKit.swift
• I have updated the SwiftPM/CocoaPods version snippets in platforms/swift/README.md (major version only)

Releasing a new Embedded Checkout Protocol version?

• I have bumped embeddedCheckoutProtocolAndroid in platforms/android/gradle/libs.versions.toml
• I have updated protocol/languages/kotlin/embedded-checkout-protocol/api/embedded-checkout-protocol.api if the public API changed

Releasing a new Android version?

• I have bumped checkoutKitAndroid in platforms/android/gradle/libs.versions.toml
• I have updated the Gradle/Maven version snippets in platforms/android/README.md

Tip

See the Contributing documentation for the full release process per platform.

[markmur]

• Abstract customer accounts api to ShopifyCustomerAccounts #512
• Migrate Android sample to AuthTabIntent #513
• Migrate Swift sample from WKWebView to ASWebAuthenticationSession #511 👈 (View in Graphite)
• main

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