Follow-up: Add browser regression test for public-asset false positive in missing media detection

## Context

Identified as a follow-up during review of PR #11873.

## Problem

The bug fixed in PR #11873 is a false positive: a public asset showing as missing in the missing media panel. Unit tests were added for the new code paths, but there is no browser/e2e regression test that loads a workflow referencing a public asset and asserts it does **not** appear in the missing media list.

`browser_tests/tests/propertiesPanel/errorsTabMissingMedia.spec.ts` already has the relevant scaffolding.

## Proposed Work

Add a browser regression test in `browser_tests/tests/propertiesPanel/errorsTabMissingMedia.spec.ts` that:
1. Loads a workflow referencing a public asset.
2. Mocks `GET /api/assets?include_tags=input&include_public=true` to return the referenced asset.
3. Asserts that the missing media group is absent (the asset is not shown as missing).

This covers the actual bug regression at the observable UI level.

## References

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11873
- Requested by: @jaeone94

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11896-Follow-up-Add-browser-regression-test-for-public-asset-false-positive-in-missing-med-3566d73d36508137a931ed5d26a5f9ec) by [Unito](https://www.unito.io)