test(assets): add partial-success mock coverage for delete confirmation flow

## Context

This is a follow-up to PR [#10785](https://github.com/Comfy-Org/ComfyUI_frontend/pull/10785) (raised in [this review comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/10785#discussion_r3019178045)).

## Problem

The `mockDeleteHistory()` helper in `browser_tests/fixtures/helpers/AssetsHelper.ts` currently models only full success — it removes all requested IDs and always returns HTTP 200. This means E2E tests cannot yet cover the partial-success / mixed-failure delete scenario (warning toast on partial failure) described in issue #10781.

The underlying reason for the deferred work: `deleteItem` in `useMediaAssetActions.ts` issues **individual** `POST /api/history` calls per asset via `Promise.allSettled`, so simulating partial failures requires per-request conditional responses from the Playwright route handler, which adds significant complexity beyond what is needed for this PR.

## Proposed work

- Extend `mockDeleteHistory()` (or add a sibling helper) to support per-request conditional responses (e.g., an optional `resolveDelete` callback or a `failingIds` set) so individual calls can be made to fail while others succeed.
- Add E2E tests asserting:
- A **warning toast** is shown when some deletes succeed and some fail.
- Asset cards for failed deletes remain visible after the operation.
- Ensure the new helper still supports the simple full-success path used by existing tests.

## References

- PR #10785 — initial delete-confirmation E2E tests
- Issue #10781 — original test coverage requirements (partial success scenario listed there)

/cc @dante01yoon

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