test: add cloud-specific pipeline unit tests for missing-model pipeline with isCloud mocking strategy

## Follow-up from PR #11751

**Context:**
PR #11751 (https://github.com/Comfy-Org/ComfyUI_frontend/pull/11751) added `src/platform/missingModel/missingModelPipeline.ts` and its test file `src/platform/missingModel/missingModelPipeline.test.ts`. Cloud-specific pipeline unit tests were deferred because they require a separate `isCloud` mocking strategy.

**Task:**
Design and implement a suitable `isCloud` mocking strategy for unit tests, then add cloud-specific unit test coverage for the missing-model pipeline (e.g., cloud asset verification flow, abort-guarded behavior in cloud mode, surfacing after cloud verification).

**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-11755-test-add-cloud-specific-pipeline-unit-tests-for-missing-model-pipeline-with-isCloud--3516d73d3650815babc9da993f6a2cb7) by [Unito](https://www.unito.io)