fix(billing): preserve dismissed payment toast across status transition

Dismissed payment toast reappears after the state transition.

Full context for agent readers

## Problem / Goal

In the billing operation flow inherited from #14910, dismissing the “Verify your payment” toast does not persist across the later transition where `action_url` clears. The UI then shows “Processing payment…” again. Preserve the user's dismissal through that transition.

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

## Proposed Solution

Track dismissal for the active billing operation independently from the current toast variant, and suppress the informational replacement until a new operation begins or the operation reaches a terminal state.

## Acceptance Criteria

- Dismissing “Verify your payment” prevents “Processing payment…” from reappearing for the same operation when `action_url` clears.
- A new billing operation can show its own status toast.
- Focused tests cover the warn-to-info transition after dismissal.