test(gateway): cover trace id tieouts

[XnLemon]

Objective

Close the Phase2 trace tie-out coverage gap for gateway inbound handling.

Completed changes

• Extends gateway usage-record test coverage so the same request/trace ID is asserted across result, outbound reply, usage record, audit record, message events, and queued outbox message.
• Switches the test to the outbox-backed outbound store so the persisted reply path is part of the tie-out.

Validation

• gofmt -w platform/gateway/service_test.go
• go test ./platform/gateway
• go vet ./platform/gateway
• git diff --check
• open-code-review subagent review: 0 High / 0 Medium / 0 Low. OCR CLI was installed, but LLM test was not configured, so strict manual review was used.

Risks / limitations

• Test-only change; no production behavior change expected.

Follow-up

• Continue Phase2 closure with the next independently reviewable acceptance gap.