fix(gcp): give every data center its own DNS records

[NJona]

Workspaces resolve per data center, and so does the platform: the frontend asks <dc-id>.<codesphere.domain> for the configuration of the data center a workspace lives in. OMS only created cs.<base-domain> and its wildcard, both pointing at the first data center's gateway, so with more than one data center the second one's endpoint resolved to the first's gateway, which has no route for that host. Every browser request for it was reset — and since the frontend fetches that config before rendering anything, the whole UI failed.

EnsureDNSRecords now creates, per data center, its workspace hosting names and SSH proxy name pointing at its own public gateway and SSH proxy, plus <dc-id>.cs.<base-domain> and its wildcard pointing at its own platform gateway.

Review notes

• The per-data-center platform names are only created when there is more than one data center: a single one is the primary, which cs.<base-domain> already resolves to. So single-DC bootstraps still create exactly the same five records.
• The created records are recorded in the infra file so cleanup deletes exactly those. DeleteDNSRecordSets therefore takes the record list instead of a base domain; cleanup falls back to deriving the names for older infra files and for a cleanup driven only by --project-id.
• Verified against a live two-data-center instance: adding these two records was what made the second data center reachable.

---

Part of the oms beta bootstrap-gcp --multi-dc stack (10 PRs). Merge in order; each PR is based on its predecessor.