feat(directory): replace service attribution with publishers

[nkavian]

Summary

• Replace typed available_through Service references with optional publisher metadata containing publisher_id, name, and website_url.
• Preserve additional response fields and accept omitted or null publisher attribution.
• Keep Collection ownership and Service discovery/execution targets unchanged.
• Update Directory documentation and attribution coverage.

This removes the old typed attribution property. The publisher website is a display link, not a Service origin or API target.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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