fix(billing): prevent initial action URL toast flicker

Checkout toast can flicker warn → info → warn during initial polling.

Full context for agent readers

## Problem / Goal

When checkout returns an `initialActionUrl` but the first status poll has not yet echoed `action_url`, the toast can transition from verification warning to processing info and back to warning. Prevent this transient state mismatch from creating visible toast churn.

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

## Proposed Solution

Retain the initial action-required state through the backend consistency window until a poll explicitly establishes a different stable state or the operation terminates.

## Acceptance Criteria

- A checkout with `initialActionUrl` does not flicker warn → info → warn when the first poll temporarily omits `action_url`.
- A genuine transition away from action-required still updates the toast.
- Focused tests model the temporary omission and stable transition.