Replace CheckoutConfigurable runtime casts with a configuration requirement

[tiagocandido]

Stack

Top to bottom:

1. Preload docs and tests: Document and test preload reuse with instance appearance #780
2. This PR
3. Preload invalidation on configure: [Swift][Bug]: discard preload cache on configuration changes (matches Kotlin) #680
4. Base: [Swift][Bug]: swiftui modifiers should be instance scoped #681

What changes are you making?

Follow-up to #681. The CheckoutConfigurable modifier defaults downcast self to ShopifyCheckout and silently returned any other conformer unchanged. The protocol now requires var configuration: ShopifyCheckoutKit.Configuration { get set }, so the shared helper copies self and updates the copy's configuration without casts — modifiers work for any value-semantic conformer, and chaining, value semantics, and isolation from the global configuration are unchanged.

Two deliberate public API changes, acceptable in the 4.0 prerelease: any external conformer now gets a compile error until it stores a configuration (instead of a silent no-op), and ShopifyCheckout.configuration becomes public with a settable value. The API baseline is regenerated for both.

How to test

Run the Swift package tests. testModifiersApplyToAnyConformerWithoutCasts covers a test-local conformer receiving all five modifiers; the existing CheckoutConfigurableTests cover ShopifyCheckout capture and global-config isolation. The public API baseline check passes against the committed baselines.

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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) — no README change needed here; the modifier docs stay accurate. Preload documentation lands in the stacked PR above.

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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

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