fix(preview): reject unsafe local file URLs

[GautamSharma99]

Summary

• Create each supervisor-link test socket inside a unique temporary directory.
• Track accepted sockets and destroy them during teardown.
• Remove temporary socket directories in afterEach.
• Centralize test server creation so cleanup stays consistent.

Why

The supervisor-link tests previously used timestamp-based socket paths directly under os.tmpdir() and did not clean up the
parent path afterward. Unique temp dirs plus explicit socket cleanup reduce stale socket/collision risk across repeated or
parallel test runs.

Tests

• npm --prefix frontend test -- supervisor-link
• npm --prefix frontend test
• npm --prefix frontend run typecheck
• npm run lint
• git diff --check

[nikhilachale]

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[somewherelostt]

Thanks for contributing to Agent Orchestrator.

This PR is being picked up by the current external contributor on-call pair:

• @neversettle17-101
• @somewherelostt

If someone is already working on this, please continue as usual.
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