test: expand E2E coverage for missing-model refresh pipeline beyond existing OSS refresh path

## Follow-up from PR #11751

**Context:**
PR #11751 (https://github.com/Comfy-Org/ComfyUI_frontend/pull/11751) refactored the missing-model pipeline but E2E coverage expansion beyond the existing OSS refresh path was intentionally deferred.

**Task:**
Add additional end-to-end test coverage for the missing-model refresh pipeline, covering scenarios beyond the existing OSS refresh path (e.g., cloud verification flows, subgraph/muted-ancestor filtering, downloadable model metadata, abort behavior).

**Relevant files:**
- `src/platform/missingModel/missingModelPipeline.ts`
- `src/platform/missingModel/missingModelPipeline.test.ts`

**Requested by:** @jaeone94 in https://github.com/Comfy-Org/ComfyUI_frontend/pull/11751

┆Issue is synchronized with this [Notion page](https://app.notion.com/p/Issue-11756-test-expand-E2E-coverage-for-missing-model-refresh-pipeline-beyond-existing-OSS-refr-3516d73d36508166a498c4f55b11227d) by [Unito](https://www.unito.io)