Fix WKNavigationDelegate policy signatures

[kieran-osgood-shopify]

What changes are you making?

Fix the Swift WKNavigationDelegate policy callback signatures on CheckoutWebView so they match the current WebKit protocol requirements.

The existing methods were missing @MainActor @Sendable on the decisionHandler closure, so Swift warned that they only "nearly" matched the optional WKNavigationDelegate requirements. That means WebKit may not dispatch navigation action/response policy callbacks to the SDK at runtime, even though direct unit tests that call the methods still pass.

This also enables warnings-as-errors for the shipped Swift package targets only:

• ShopifyCheckoutProtocol
• ShopifyCheckoutKit
• ShopifyAcceleratedCheckouts

That keeps this guardrail focused on our library code instead of test-only dependencies like ViewInspector. To use SwiftPM's first-class .treatAllWarnings(as: .error) setting, the manifest tools version is bumped from 6.0 to 6.2.

Why?

The checkout HTTP error handling path depends on webView(_:decidePolicyFor:decisionHandler:) for navigation responses calling handleResponse(_:). If WebKit does not see the selector as implemented, 4xx/5xx checkout responses can render or leave the sheet blank instead of flowing through the SDK's intended error handling.

Existing tests did not fail because they call handleResponse(_:) or the Swift methods directly, bypassing Objective-C optional-protocol selector dispatch. This PR adds a regression test that checks the CheckoutWebView instance responds to the WebKit policy selectors.

Enabling warnings-as-errors on the package library targets would have caught the original warning at build time while avoiding noise from test targets and third-party test dependencies.

How to test

• shadowenv exec -- env CURRENT_SIMULATOR_UUID=48B8B315-0429-4EB2-A482-F5223CBD96D7 platforms/swift/Scripts/xcode_run test ShopifyCheckoutKit-Package CheckoutWebViewTests
• shadowenv exec -- env CURRENT_SIMULATOR_UUID=48B8B315-0429-4EB2-A482-F5223CBD96D7 /opt/dev/bin/dev swift build packages
• shadowenv exec -- /opt/dev/bin/dev swift lint
• shadowenv exec -- env CURRENT_SIMULATOR_UUID=48B8B315-0429-4EB2-A482-F5223CBD96D7 /opt/dev/bin/dev swift test

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

Releasing a new Android version?

• I have bumped the versionName in platforms/android/lib/build.gradle
• I have updated platforms/android/CHANGELOG.md
• 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.

[kieran-osgood-shopify]

• Fix WKNavigationDelegate policy signatures #320 👈 (View in Graphite)
• main

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