fix(server): default supported_billing: [] in createAdcpServerFromPlatform (match v5 behavior)

[bokelley]

Closes #1186

Summary

When requireOperatorAuth is true (or derived from accounts.resolution: 'explicit') but supportedBillings is not set, the v6 createAdcpServerFromPlatform path projected the account block to the wire response but omitted supported_billing. The AdCP schema requires the field whenever account is present, so schema validation failed with missing required property 'supported_billing' — and the storyboard runner cascade-failed every subsequent step.

Root cause: src/lib/server/decisioning/runtime/from-platform.ts:767 used a conditional spread ...(supportedBillings?.length && { supported_billing: [...] }) that produced nothing when supportedBillings was undefined or empty. The v5 createAdcpServer path at create-adcp-server.ts:3431 correctly defaulted to supported_billing: capConfig.account.supportedBilling ?? [].

Fix: Replace the conditional spread with supported_billing: supportedBillings?.length ? [...supportedBillings] : [] — same one-line change the v5 path uses. Also corrects the DecisioningCapabilities.supportedBillings JSDoc which claimed the default was ['agent'] (it was always []).

What was tested

• npm run format:check — passed
• npx tsc --project tsconfig.lib.json — passed (pre-existing TS2688/TS5107 env errors unrelated to this change)
• node --test test/server-decisioning-capability-projections.test.js — 13/13 pass (new regression test is test Fix A2A agent card validation endpoint #12)
• Full suite: 4542 tests, 4278 pass, 220 fail — baseline has 222 fail; no new failures introduced
• New regression test uses validation: { responses: 'strict' } to exercise the actual schema-validation path that the bug broke

Nits noted (not fixed in this PR):

• The accounts.resolution: 'explicit' path in the pre-existing test (line 210) uses responses: 'off' — a follow-up could upgrade it to responses: 'strict' + assert supported_billing: []. The fix is correct for both paths since the logic is shared; this is a coverage gap, not a correctness gap.
• docs/llms.txt and skills/build-retail-media-agent/SKILL.md have zero mentions of supportedBillings — doc improvement tracked in issue.

Pre-PR review

• code-reviewer: approved — no blockers; fix is minimal and correct; test correctly exercises schema validation with responses: 'strict'
• dx-expert: approved — no blockers; JSDoc correction is accurate; changeset bump (patch) is appropriate

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
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• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01CbCVxFuiFnDHrBFwNFbWMs

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[bokelley]

Triage: superseded by #1198

Issue #1186 was closed by #1198 (commit 14f011c7), which shipped a different default: ['agent'] (not []).

The schema requires supported_billing with minItems: 1 (schemas/cache/3.0.1/protocol/get-adcp-capabilities-response.json), so emitting [] would fail schema validation. #1198's ['agent'] default matches the platform-interface JSDoc contract at capabilities.ts:130 ("Defaults to ['agent'] when omitted") and is the least-surprising default for non-media-buy specialisms.

Closing this as superseded — the working fix is already on main.

[bokelley]

Acknowledged — thanks for the context, @bokelley. Noted that #1198 is the authoritative fix on main with ['agent'] as the default (matching minItems: 1 schema requirement and the JSDoc contract at capabilities.ts:130). No further action from this session.

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