Refresh CheckoutController state after confirmation

[jaynewstrom-stripe]

Summary

• Adds a confirmation-state updater that uses the response returned by synchronous confirmation or refetches after a next action.
• Reloads all response-derived controller state before delivering Completed.
• Serializes confirmation refreshes with configuration and mutation work while keeping refresh failures from changing a successful payment result.

At this layer the refreshed session remains loaded; the final PR clears it after refresh.

Stack position: 4 of 5, based on #13802.

Motivation

Confirmation can finish while another controller mutation is still in flight, and next-action results do not carry a Checkout Session response. Refreshing through the controller's existing serialization path prevents stale writes and ensures derived state is synchronized without converting a completed payment into a failure when refresh work fails.

Testing

• Added tests
• Modified tests
• Manually verified

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

No tests were ignored.

Screenshots

Not applicable — there are no visual changes.

Changelog

Not included — CheckoutController remains a preview API.