feat(domains): add TrackingCAA record support

[rehanvdm]

Summary

The Domains API now returns an additional DNS record on POST /domains and GET /domains/:id responses when:

1. A tracking_subdomain is configured on the domain, AND
2. The customer's root domain has CAA records that would otherwise prevent AWS from issuing a TLS certificate.

The new record looks like:

{
  "record": "TrackingCAA",
  "name": "",
  "type": "CAA",
  "ttl": "Auto",
  "value": "0 issue \"amazon.com\"",
  "status": "verified"
}

DomainRecord.Record is already a free-form string so no source change is required for deserialization. This PR:

• Updates the xmldoc remark on DomainRecord.Record to list TrackingCAA as a possible value.
• Adds a Domain_deserializes_tracking_caa_record test that exercises both Tracking and TrackingCAA records end-to-end.

Test plan

• New unit test deserializes a domain payload containing both Tracking and TrackingCAA records and asserts every field
• CI runs full test suite

---

Summary by cubic

Add support for the TrackingCAA DNS record in Domains API responses when a tracking subdomain is set and existing CAA records would block AWS certificate issuance. No runtime changes needed; updated DomainRecord.Record remarks to include TrackingCAA and added a unit test that deserializes both Tracking and TrackingCAA records.

^{Written for commit f058726. Summary will update on new commits.}

[cubic-dev-ai]

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