fix(billing): shorten stale verification prompt after bank challenge

Payment verification prompt can remain stale for 30 seconds.

Full context for agent readers

## Problem / Goal

On `core/1.49`, once `authenticationRequiredSeen` is set, billing status polls use the 30-second action-required cadence. After a customer completes the bank challenge, “Verify your payment” can therefore remain visible for up to 30 seconds. Reduce stale prompt time without backporting the larger embedded-checkout consolidation.

Source: https://github.com/Comfy-Org/ComfyUI_frontend/pull/15674#discussion_r3858680007

## Proposed Solution

Introduce the smallest isolated cadence rule needed to poll promptly after customer action, while preserving the slower parked-state cadence and avoiding the full `isParkedAwaitingCustomer` consolidation.

## Acceptance Criteria

- Completing the bank challenge clears or replaces the verification prompt promptly rather than after the 30-second interval.
- Parked billing operations retain a bounded polling cadence.
- Focused timer tests cover the post-challenge transition.