feat(decisioning): boot-time validate_capabilities_response_shape against spec invariants

[bokelley]

Context

protocol/get-adcp-capabilities-response.json requires account.supported_billing when seller claims media_buy. The framework's projection wasn't checking this, so the v3 ref seller was non-conformant until PR #402 fixed it manually.

Add a boot-time validator that exercises the capabilities response shape against the spec; refuse to start if the projection violates an invariant.

Acceptance Criteria

• New validate_capabilities_response_shape() invoked from create_adcp_server_from_platform
• Failing platform fails fast at boot with a descriptive error
• Test covers account.supported_billing invariant when media_buy is claimed

Refs

• PR feat(v3-ref-seller): enable server validation + declare account.supported_billing #402 (manual fix for account.supported_billing)
• protocol/get-adcp-capabilities-response.json

[bokelley]

Triage

Classification: Feature request
Bucket(s): validation, handlers
Status: drafting-pr
Milestone: omitted

What the experts said:

• ad-tech-protocol-expert: spec invariant is real (spec prose says "should"; SDK enforces it as a hard boot gate — same precedent as the governance opt-in gate); all media_buy-mapped specialisms covered correctly
• code-reviewer: approved after fix — use AdcpError("INVALID_REQUEST", recovery="terminal") to match existing validators; add __all__ entry and a serve-layer integration test
• dx-expert: fail-fast is the right UX; error message should include what failed, what spec requires, and how to fix it; standalone callable preferred for testability

My take: Clean non-breaking addition — create_adcp_server_from_platform already has multiple fail-fast validators; this adds a sixth that enforces a pre-existing spec requirement. Conformant platforms (already have supported_billing) are unaffected. Draft PR #432 implements validate_capabilities_response_shape() in dispatch.py, calls it after validate_platform() in serve.py, and ships 5 new tests (4 unit + 1 serve-layer integration).

One-line PR summary: Add validate_capabilities_response_shape() boot-time invariant check — raises AdcpError("INVALID_REQUEST") when media_buy-mapped specialism is claimed without supported_billing.

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Triaged by Claude Code. Session: https://claude.ai/code/session_01R5BFr24QJGTr9RW74biz7D

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