fix(server): tighten handler return types so schema drift fails tsc

[bokelley]

Summary

Closes #727 (B). Makes sparse handler returns like { rights_id, status: 'acquired' } fail tsc at compile time against AcquireRightsResponse, instead of only failing at wire-level validation.

Two hand-written type bindings in create-adcp-server.ts were routing around the (already-tight) generated discriminated unions:

• AdcpToolMap brand-rights slots were typed result: Record<string, unknown> — stale scaffold from before the response types were code-generated. Swapped for the generated success types (AcquireRightsAcquired | AcquireRightsPendingApproval | AcquireRightsRejected, GetRightsSuccess, GetBrandIdentitySuccess).
• DomainHandler's return union included | Record<string, unknown> as a general escape hatch for every tool. Removed. Handler returns are now AdcpToolMap[K]['result'] | McpToolResponse — adcpError(...) still works unchanged.

No upstream schema or generator changes needed. The looseness was purely in SDK plumbing; the generated types were already correct discriminated unions with proper required arrays.

What this surfaced

Removing the escape hatch caught two real latent bugs in the reference test agents:

• test-agents/seller-agent.ts: PRODUCTS were missing reporting_capabilities (required on Product). Strict response validation (default in dev/test per feat(server): strict response validation default in dev/test (#727) #757) would have caught this at runtime; now it fails at build. Fixed with DEFAULT_REPORTING_CAPABILITIES.
• test-agents/seller-agent-signed-mcp.ts: createMediaBuy was reading pkg.package_id from the request, but PackageRequest has no such field per spec — buyers send buyer_ref and the seller mints package_id. Fixed to mint crypto.randomUUID().

Regression guard

test/handler-return-tightness.test.js asserts (a) no brand-rights slot in AdcpToolMap is typed Record<string, unknown> and (b) DomainHandler's return union does not contain Record<string, unknown>. The runtime suite alone can't catch a reintroduced compile-time escape hatch.

Test plan

• npm run build:lib — clean
• npm run build:test-agents — clean (was 2 errors before the test-agent fixes)
• npm run test:lib — 4391 pass, 0 fail
• Verified with @ts-expect-error harness: sparse shapes, empty objects, and { anything: 'goes' } all fail tsc at BrandRightsHandlers['acquireRights'] return position; valid shapes compile
• code-reviewer + security-reviewer feedback addressed

Migration

Minor bump. If a handler returns a plain object literal without the full success shape, tsc will now flag it. Either fill in the missing required fields (use DEFAULT_REPORTING_CAPABILITIES for Product.reporting_capabilities) or wrap with a response builder (productsResponse({ ... }), acquireRightsResponse({ ... })) — details in the changeset.

🤖 Generated with Claude Code