Adds three functions used by replicated-actions to surface airgap build failures

[bennyyang11]

Summary

Adds three airgap-build helpers used by replicated-actions#403 (sc-136443) to surface airgap build failures after a release promotion:

• getAirgapBuildStatus — polls the dedicated /airgap/status endpoint for a single channel-release. Returns null on 404.
• getLatestAirgapStatusForRelease — finds the latest channel-release for a given release sequence (handles multi-promote correctly).
• getAirgapBundleDownloadURL — variant of getDownloadUrlAirgapBuildRelease that takes a pre-known channelSequence, skipping the redundant /releases scrape.

Also extends ChannelRelease with airgapBuildError and adds an AirgapBuildStatus interface matching the new wire shape from vandoor#9761.

Test plan

• npm run build — clean
• npx jest channels.spec — 15/15 passing (9 new mockttp-backed tests covering 200 / pending / 404 / 5xx / multi-promote / no-match / no-releases-array)
• Once merged + published, replicated-actions#403 will be updated to import from this lib instead of inlining the HTTP calls (per JD's review)