Route Embedded saved payment method selection through a handler

[cttsai-stripe]

Summary

Embedded saved payment method selection still commits the selection immediately before invoking the existing row-selection callback.

This PR moves that sequence behind an internal, select-only EmbeddedSavedPaymentMethodSelectionHandler. The Embedded vertical-layout factory delegates saved method selection to the handler, while callers without a handler retain the existing direct selection path.

What changed

• Added the internal saved payment method selection handler.
• Bound its default implementation in the standard Embedded and Checkout component graphs.
• Routed the production Embedded vertical-layout factory through the handler.
• Added a required nullable handler callback to the shared vertical-layout interactor.

What stays the same

• The default handler commits the selection before invoking the callback.
• PaymentSheet and sheet-based callers continue to update selection and invoke the callback directly.
• This PR adds no pending state, error state, spinner UI, or Checkout controller behavior.
• No public API changes are introduced.

Motivation

Checkout needs to perform work before completing saved payment method selection. Establishing the routing boundary separately keeps the behavior change in #14340 focused while preserving existing Embedded behavior here.

Testing

• Added tests
• Modified tests
• Manually verified

Coverage verifies that the default handler commits before invoking the callback, that the interactor delegates without duplicating the update or callback, and that the existing fallback behavior remains unchanged.

Focused handler and interactor tests passed. The path-aware pre-push hook passed :paymentsheet:apiCheck and :paymentsheet:detekt.

CI: pending.

Screenshots

Not applicable. This routing refactor has no UI changes.

Committed and created by Codex.