feat(schema): bills_through_adcp capability + BILLING_OUT_OF_BAND error (closes #2881, #2882)

[bokelley]

Summary

Two strictly additive schema changes spawned by #2879 (which softened creative-ad-server conformance for agents that bill out of band). Both close loops that #2879 deliberately left open.

• capabilities.creative.bills_through_adcp (boolean, default false/absent) on get-adcp-capabilities-response.json. Pre-call discriminator so buyer agents can pre-filter creative agents across a portfolio before establishing an account just to probe pricing. When true, buyers can expect pricing_options on list_creatives, pricing_option_id/vendor_cost on build_creative, and report_usage that accepts records. Closes spec: add bills_through_adcp capability to creative get_adcp_capabilities #2881.

• BILLING_OUT_OF_BAND (recovery: terminal) on the error-code enum. The standard per-record report_usage rejection for accounts that bill via a non-AdCP channel. Distinct from BILLING_NOT_SUPPORTED (media-buy billing-value rejection) and BILLING_NOT_PERMITTED_FOR_AGENT (per-buyer-agent commercial gate) — this signals that the entire billing surface is offline for the account, not that a specific value or caller is rejected. The code itself is the discriminator; no error.details shape defined (mirroring CONFIGURATION_ERROR). Closes spec: standard error code for "agent does not accept usage records" #2882.

Investigation note: checked whether OPERATION_NOT_SUPPORTED existed as #2882 raised — it does not. BILLING_OUT_OF_BAND is the right choice on specificity grounds and not redundant.

Both follow the established 3.0.x drift policy: held-for-next-minor / 3.1. Adding an enum value is a wire change and ships in 3.1, not as a 3.0.x patch.

What this touches

• static/schemas/source/protocol/get-adcp-capabilities-response.json — bills_through_adcp added to the creative block alongside the existing boolean siblings.
• static/schemas/source/enums/error-code.json — BILLING_OUT_OF_BAND in enum, enumDescriptions, and enumMetadata (per the dual-surface pattern established in feat(schema): manifest.json + structured enumMetadata (closes #3725) #3738).
• scripts/error-code-drift-dispositions.json — held-for-next-minor / 3.1 entry.
• static/compliance/source/specialisms/creative-ad-server/index.yaml — report_usage narrative replaces the "vendor codes are fine today" placeholder with the standard code. No new hard assertions (the skip_if gating on the new capability is a separate follow-up — keeping fix(storyboards): make creative-ad-server pricing/billing optional #2879's softer conformance posture).
• specs/creative-agent-pricing.md — Pre-account-discovery and Capabilities-change sections updated.
• docs/protocol/get_adcp_capabilities.mdx — capability table row + example.
• .changeset/2881-2882-creative-billing-schema.md

Test plan

• npm run test:error-codes clean (87 storyboard files scanned against 67 canonical codes — +1)
• npm run test:error-code-drift clean (22 ahead, all dispositioned)
• npm run test:schemas — 525 schemas, 7 tests passed
• npm run test:json-schema — 260 schema blocks validated
• npm run test:storyboard-doc-parity clean
• npm run test:examples clean
• npm run test:unit — 887 tests pass
• npm run typecheck clean
• CI green

Backward compatibility

Strictly additive. No existing agents break — they remain in probe-to-discover mode until they opt into declaring bills_through_adcp. Buyers that haven't upgraded continue to probe; the response shape for report_usage is unchanged, only the errors[].code vocabulary is widened.