Deliver CheckoutController confirmation results

[jaynewstrom-stripe]

Summary

• Adds a controller-scoped handler that converts terminal confirmation states into CheckoutController.Result callbacks.
• Delivers completed and failed results, reports applicable cancellations, and handles interrupted next actions after process death.
• Registers result observation when the controller is constructed and updates the playground to use Result.Completed for completion.

Stack position: 3 of 5, based on #13801.

Motivation

CheckoutController.Builder.resultCallback was bound into the component but terminal confirmation results were not forwarded to it. Centralizing the mapping gives the controller one exactly-once result path and preserves the distinction between merchant-visible cancellation and returning to payment details.

Testing

• Added tests

• Modified tests

• Manually verified

• ./gradlew :paymentsheet:testDebugUnitTest --tests com.stripe.android.checkout.CheckoutConfirmationResultHandlerTest --tests com.stripe.android.checkout.CheckoutControllerTest

• ./gradlew :paymentsheet-example:compileBaseDebugKotlin

No tests were ignored.

Screenshots

Not applicable — there are no visual changes.

Changelog

Not included — CheckoutController remains a preview API.