Add WebCheckoutOpened result & handling

[alexrepty]

Introduce WebCheckoutOpened to represent the user tapping a web checkout CTA and leaving the app. Add PaywallListener.onWebCheckoutOpened(), propagate a notifyWebCheckoutOpened() call through PaywallViewModel (impl calls listener), and have InternalPaywall call it and close the paywall with PaywallResult.WebCheckoutOpened when autoDismiss is true. Add a no-op override in LoadingViewModel, update MockViewModel to track calls/results, forward the event to user listeners in PaywallActivity, and add a test ensuring LaunchWebCheckout with autoDismiss closes with WebCheckoutOpened. Update api.txt to expose the new API surface.

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-ios and hybrids

Motivation

Description

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Note

Low Risk
Additive public API and clearer dismiss semantics for web checkout; limited to RevenueCat UI paywall flows with tests.

Overview
Adds a dedicated web checkout outcome so hosts can tell “user left for external payment” apart from cancelled or a completed in-app purchase.

PaywallResult.WebCheckoutOpened is a new parcelable activity result. PaywallListener.onWebCheckoutOpened() fires after the checkout URL opens. PaywallViewModel.notifyWebCheckoutOpened() routes that to the listener; on LaunchWebCheckout with autoDismiss, InternalPaywall now calls closePaywall(PaywallResult.WebCheckoutOpened) instead of a generic dismiss. PaywallActivity forwards the listener callback; loading/mock view models implement the new API; api.txt and a compose test cover auto-dismiss + result.

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[RevenueCat-Danger-Bot]

	1 Error
🚫	Label the PR using one of the change type labels. If you are not sure which label to use, choose pr:other.

Label	Description
pr:feat	A new feature. Use along with pr:breaking to force a major release.
pr:fix	A bug fix. Use along with pr:force_minor to force a minor release.
pr:other	Other changes. Catch-all for anything that doesn't fit the above categories. Releases that only contain this label will not be released. Use along with pr:force_patch, or pr:force_minor to force a patch or minor release.
pr:RevenueCatUI	Use along any other tag to mark a PR that only contains RevenueCatUI changes
pr:next_release	Preparing a new release
pr:dependencies	Updating a dependency
pr:phc_dependencies	Updating purchases-hybrid-common dependency
pr:changelog_ignore	The PR will not be included in the changelog. This label doesn't determine the type of bump of the version and must be combined with pr:feat, pr:fix or pr:other.

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