fix(conformance): treat absent proposal support as unsupported

[bokelley]

Summary

• Treat omitted media_buy.supports_proposals as unsupported for proposal-lifecycle requires_capability gates.
• Apply the same optional-capability exception when a profile has no raw capabilities because get_adcp_capabilities is unavailable.
• Add regression coverage for absent, true, and false proposal support gate behavior.

Fixes #2245.

Validation

• npm run build:lib
• NODE_ENV=test node --test-timeout=60000 --test-force-exit --test test/lib/storyboard-capability-gate.test.js
• npx prettier --check src/lib/testing/storyboard/runner.ts src/lib/testing/storyboard/types.ts test/lib/storyboard-capability-gate.test.js
• git diff --check
• pre-push validation: npm run typecheck + build hook

[aao-ipr-bot]

Right fix, and it's spec-mandated rather than a judgment call. Treating absent supports_proposals as unsupported puts it on the same opt-in trust-boundary footing as inline_creative_management — the seller advertises the feature, or the conformance runner skips its storyboard.

Things I checked

• Spec authority. media_buy.supports_proposals's own schema description (core.generated.ts:22594-22597) reads "When false or absent, conformance runners skip proposal-lifecycle storyboards." Absence semantics are pre-declared by the field author — this is not the ambiguous "agent may pre-date the field" case the general run-anyway rule exists for. ad-tech-protocol-expert: sound, spec-cited.
• Type-predicate widening. isAbsenceMeansUnsupportedGate narrows to { path: string; equals: true } instead of the inline literal. Both call sites (runner.ts:345, runner.ts:377) feed predicate straight into evaluateCapabilityPredicate, which takes the full union and never reads path as a literal — no caller depended on the narrowed type. Safe.
• Symmetric coverage. Both the raw_capabilities-present path and the no-get_adcp_capabilities profile path route through the same Set, so the two exception paths can't drift.
• Absence vs. explicit-false. The actual !== undefined && actual !== equals branch is preserved, so explicit supports_proposals: false still skips. New test at storyboard-capability-gate.test.js exercises undefined→skip, true→run, false→skip, plus the raw-caps-unavailable profile.
• Changeset. Present, patch, correct shape — conformance-runner gating behavior only, no public API or signature change. code-reviewer: patch matches impact.

Follow-ups (non-blocking — file as issues)

• sales-proposal-mode is a stable specialism. A seller that genuinely sells via proposals but forgets the flag now silently gets proposal storyboards skipped rather than failed. That's the deliberate opt-in-is-the-trust-boundary design, and the schema's "buyers should still honor any proposals the seller actually returns" clause is the runtime safety net — but worth a one-line note in the adopter-facing validate guide so the skip doesn't read as a false pass.
• Per the spec→mock→SDK triage order, the SDK doc-comment is generated downstream. Worth confirming the canonical adcontextprotocol/adcp media-buy capability prose carries the identical "false or absent → skip" language. The generated schema is strong evidence; the spec repo is the source of truth.

LGTM. Follow-ups noted below.