fix(api): align marketplace download-url references in unit test and e2e helpers

[AlexMultiAgent]

fix(api): align marketplace download-url references in unit test and e2e helpers

Fixes #39892

Summary

• Assert the exact renamed endpoint in test_get_plugin_pkg_url_contains_unique_identifier and add a negative assertion for the old path.
• Update the E2E marketplace seed helper to download from /api/v1/plugins/download-url.
• Update the E2E test fixture message to the new URL; the fallback matcher still matches both old and new error text.

Root cause

PR #39811 renamed the backend marketplace download URL from /api/v1/plugins/download to /api/v1/plugins/download-url but did not update the E2E seed helper or the backend unit test. The unit test kept passing only because the new path contains the old path as a substring.

Validation

• Backend: test_marketplace.py passes and now pins the exact path.
• E2E: marketplace-plugins.test.ts still exercises the fallback branch with the updated fixture.

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods

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