[Bug] E2E seed helpers and a unit test still reference the old /api/v1/plugins/download endpoint after it was renamed to /download-url

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### Dify version

`main` @ `b6824334cf`

### Cloud or Self Hosted

- [x] Self Hosted (Source)

### Steps to reproduce

1. Commit `69a16d83` (#39811) renamed the marketplace plugin download endpoint in the backend helper:

`api/core/helper/marketplace.py:22` now builds `/api/v1/plugins/download-url`.

2. The E2E marketplace seed helper still downloads from the old path:

`e2e/support/marketplace-plugins.ts:103` → `new URL('/api/v1/plugins/download', ...)`.

3. The backend unit test still only asserts the old path:

`api/tests/unit_tests/core/helper/test_marketplace.py:20` → `assert "api/v1/plugins/download" in url`.

### ✔️ Expected Behavior

All references in the repository use the renamed endpoint. The unit test verifies the exact new path, and the E2E fallback downloads from `/api/v1/plugins/download-url`.

### ❌ Actual Behavior

The E2E fallback path (`downloadMarketplacePluginPackage`) requests the old endpoint; if the marketplace stops serving `/plugins/download`, the fallback returns 404 and seeding falls back to local package upload with a confusing error. The unit test still passes only because `"download-url"` contains the substring `"download"`, so it does not actually verify the renamed endpoint and would not catch a future rename.

Start with api/core/helper/marketplace.py, e2e/support/marketplace-plugins.ts:103, and api/tests/unit_tests/core/helper/test_marketplace.py:20. Update the E2E fallback and make the unit assertion verify /api/v1/plugins/download-url rather than the old path. Run the affected unit test and verify the marketplace seed fallback uses the renamed endpoint.

Written by the indexing model from the issue text.