[Swift][Bug]: Add missing ecp params to checkout URLs in CheckoutViewController

[kieran-osgood-shopify]

TLDR

CheckoutViewController is public. It is missing the ecp query params so it never subscribes to protocol.

All API's route through CheckoutViewController, and decorate the url before instantiating it

This PR moves the decoration logic into CheckoutViewController, so all paths are covered equally.

Fixes shop/issues-checkout-kit#1215.

What changes are you making?

This change:

• decorates URLs in the public and accelerated-checkout CheckoutViewController initializers
• removes redundant decoration from UIKit convenience presentation and SwiftUI
• preserves preload as its own decoration boundary
• adds coverage for every URL entrypoint, managed query parameter replacement, and idempotence
• lets a directly constructed CheckoutViewController reuse a checkout preloaded from the same undecorated URL, because both now resolve to the same decorated URL

How to test

The Swift demo app's CheckoutDelegate.checkoutDidStart(_:) handler logs [CheckoutKitSwiftDemo] Started: <checkout id>. Checkout Kit calls it only when it receives the Embedded Checkout Protocol ec.start notification, so the Started: log line means ec.start was received.

GIVEN the Swift demo app is running from this PR's branch on a simulator

AND Settings → Features → Checkout presentation is set to UIKit, so the Cart tab presents a directly constructed CheckoutViewController

AND Settings → Features → Checkout preloading is disabled

WHEN you add a product, open the Cart tab and tap Check out

THEN the Xcode console logs [CheckoutKitSwiftDemo] Started: gid://shopify/Checkout/… once checkout loads, confirming that ec.start was received

AND checkout opens without the store header or the Information › Shipping › Payment › Review breadcrumbs

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GIVEN the Swift demo app is running from the base branch (#775) on a simulator

AND Settings → Features → Checkout presentation is set to UIKit

AND Settings → Features → Checkout preloading is disabled

WHEN you add a product, open the Cart tab and tap Check out

THEN checkout loads, but the console never logs Started:, because ec.start is never received

AND checkout shows the store header and the step breadcrumbs

Closing checkout logs Dismissed on both branches, because it comes from the native close button rather than the protocol.

No order needs to be placed.

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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)

README changes are not required because the documented presentation APIs and usage remain unchanged.

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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.

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[bitrise]

Bitrise builds

E2E · iOS CI

Install this build

Open Tophat, select your target device, then click Install. Links open on the Mac running Tophat.

SDK	Install
Swift	Install with Tophat

Checkout Kit E2E results

Status	Tags	Target	Platform	OS version tag	Device
✅	launch, checkout-presentation, checkout-completion, buyer-identity, preload	swift	ios	latest	iPhone 15 iOS 27 Beta