fix(compliance): restore available actions patch changeset

[bokelley]

Summary

• replace the empty changeset merged in compliance(media-buy): available_actions should be runnable by sales-non-guaranteed-only sellers #5731 with the required adcontextprotocol patch declaration
• strengthen the source comment preserving the non-guaranteed fixture invariant

#5731 correctly fixed the false compliance failure, but its changeset frontmatter remained empty. Empty padding changesets are disallowed by the repository playbook and would omit the fix from the next protocol patch release; this follow-up repairs that release metadata immediately.

Validation

• git diff --check
• changeset scope now includes the compliance source asset
• compliance(media-buy): available_actions should be runnable by sales-non-guaranteed-only sellers #5731 functional change had a fully green CI matrix before merge

[aao-secretariat]

Argus is not auto-reviewing this PR because it modifies protected paths that require human review (.changeset/available-actions-non-guaranteed.md). A human reviewer should review and merge this PR; Argus will resume on subsequent PRs once these changes land on main.