fix(monetize): drop available, use drainEndsAt as sole drain signal

[bussyjd]

Summary

drain becomes purely additive — active offers serialize identically to pre-drain main. The only new wire field is drainEndsAt, set on draining offers only. Consumers detect drain with if (entry.drainEndsAt) { /* draining */ }. No schema-breaking change for any consumer that was reading the catalog before drain landed.

This re-ships an amendment that was originally pushed as commit dd89750 on feat/drain-replaces-pause for PR #535. The amendment didn't survive the bundle PR #536's merge into main, so the controller is shipping the un-amended Available bool shape today.

Diff summary

• internal/schemas/service_catalog.go — remove Available bool from ServiceCatalogEntry; keep DrainEndsAt string omitempty`` as the sole drain marker.
• internal/schemas/service-catalog.schema.json — drop available from required and from properties; tighten the drainEndsAt description to "present iff draining".
• internal/serviceoffercontroller/render.go
  • buildServiceCatalogJSON: stop setting Available; only set DrainEndsAt when offer.IsDraining().
  • buildSkillCatalogMarkdown: rename the Available table column to Status (active rows show —; draining rows show draining · ends `<RFC3339>` ); drop the per-service - **Available**: bullet entirely (draining services keep only - **Drain ends at**:).
  • serviceDefWithDrain: stop setting the (already-additive) Available *bool on erc8004.ServiceDef during drain — signal drain via DrainEndsAt only.
• internal/serviceoffercontroller/render_test.go
  • TestBuildServiceCatalogJSON_ExcludesNonReady: replace services[0].Available == true assertion with a raw-JSON map walk asserting available and drainEndsAt keys are absent on active entries.
  • TestBuildServiceCatalogJSON_DrainLifecycle: rewrite the lifecycle assertions to walk []map[string]any. Active entries: neither available nor drainEndsAt. Mid-drain: only drainEndsAt. Expired: filtered.
  • TestBuildRegistrationServices_IncludesDrainMetadata and TestBuildIdentityRegistrationServices_IncludesDrainMetadata: replace svc.Available == &false checks with svc.Available == nil (the helper no longer sets it; presence-of-DrainEndsAt is the sole drain marker).
  • NEW TestBuildSkillCatalogMarkdown_DrainAdditiveDetail: asserts no - **Available**: bullet appears for any offer, draining offers keep their - **Drain ends at**: bullet, and the table header uses Status not Available.

Test plan

• go build ./... clean
• go test ./internal/schemas/... ./internal/serviceoffercontroller/... ./internal/monetizeapi/... green
• go test ./... green (full suite, including internal/stack)
• TestBuildServiceCatalogJSON_ExcludesNonReady confirms active-offer JSON has no available key
• TestBuildServiceCatalogJSON_DrainLifecycle confirms draining offer still emits drainEndsAt (and only drainEndsAt)
• TestBuildSkillCatalogMarkdown_DrainAdditiveDetail confirms markdown surface is additive: no - **Available**: bullet, Status column header, - **Drain ends at**: retained on draining offers.