feat: clear stale missing-model state on real verification failures in cloud pipeline

## Summary

When `verifyAssetSupportedCandidates` rejects due to a **real** (non-abort) error in the cloud missing-model pipeline, the Errors tab and the workflow warning cache are left with stale state from a previous pipeline run. The `catch` handler currently only logs and toasts; it does not call `useExecutionErrorStore().surfaceMissingModels([], { silent })` or `updatePendingWarnings` to clear that state.

## Expected behaviour

On a genuine verification failure (i.e., `controller.signal.aborted === false`), the pipeline should clear both:
- the execution error store (`surfaceMissingModels([], { silent })`)
- the workflow pending-warnings cache (`missingModelCandidates: []`)

so the UI reflects the uncertainty rather than showing potentially wrong stale errors.

## Context

This was intentionally deferred from the behaviour-preserving extraction in #11751 (see [review comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/11751#discussion_r3160561317)) to avoid mixing error-state policy changes into a refactor PR. A dedicated fix here should be accompanied by targeted tests covering the failure-path behaviour.

## File

`src/platform/missingModel/missingModelPipeline.ts` — `.catch` handler on `verifyAssetSupportedCandidates` (cloud branch of `runMissingModelPipeline`).

---

_Raised by @coderabbitai as a follow-up to #11751, requested by @jaeone94._

┆Issue is synchronized with this [Notion page](https://app.notion.com/p/Issue-11752-feat-clear-stale-missing-model-state-on-real-verification-failures-in-cloud-pipeline-3516d73d36508190823be815ba2eb2f9) by [Unito](https://www.unito.io)