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

[bokelley]

Closes #422

Adds validate_capabilities_response_shape() called from create_adcp_server_from_platform after validate_platform. Raises AdcpError("INVALID_REQUEST", recovery="terminal") when any claimed specialism maps to media_buy (sales-*, audience-sync) but supported_billing is empty — the projection in PlatformHandler.get_adcp_capabilities silently drops the account block in that case, producing a wire-invalid capabilities response.

Fixes the gap exposed by PR #402 (manual supported_billing fix on the v3 ref seller): non-conformant platforms now fail fast at boot with an actionable diagnostic rather than shipping a broken capabilities wire shape. The check operates on DecisioningCapabilities fields directly (synchronous, no handler invocation, consistent with the existing validate_platform pattern).

Migration note: Existing platforms that claim any sales-* or audience-sync specialism without supported_billing will now raise at boot. The fix is a one-liner: supported_billing=["operator", "agent"] (or a subset) in DecisioningCapabilities. Platforms that were already spec-conformant see no change.

What was tested

• pytest tests/test_decisioning_dispatch.py — 4 new tests: failing path (sales-non-guaranteed + empty supported_billing), happy path (populated supported_billing), non-media_buy specialism (signals), and audience-sync edge case. All pass.
• pytest tests/test_decisioning_serve.py — 1 new integration test (test_create_propagates_capabilities_shape_failure), 1 fixture fix (_SalesPlatformWithRequiredMethods now includes supported_billing). All 19 tests pass.
• ruff check clean
• mypy src/adcp/decisioning/dispatch.py src/adcp/decisioning/serve.py clean

Pre-PR review:

• code-reviewer: approved — added __all__ entry and serve-layer integration test per review feedback
• ad-tech-protocol-expert: approved — message corrected to "SDK-enforced invariant derived from spec intent" (spec uses advisory "should" not a hard JSON Schema conditional); coverage of all media_buy-mapped specialisms confirmed correct

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See adcp#3121
for context.

Session: https://claude.ai/code/session_01R5BFr24QJGTr9RW74biz7D

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[bokelley]

Closing in favor of #446 — same issue (#422), broader implementation: bundled-schema validation, structured per-issue diagnostics, ripple-effect fixture fixes, 7 tests, full suite passes.