Verify outbound request signing with mocktioneer #217

Description

Exercise outbound requests against mocktioneer’s request-signing verification feature. The original issue does not name the mocktioneer endpoint, signing contract, adapters or expected assertions; those details still need to be recorded.

Steps to reproduce

Inspection steps (no outbound test executed):

1. Inspect current ProxyClient contract and the app-demo proxy handlers.
2. Search the repository for mocktioneer and request-signing fixtures.
3. No checked-in fixture documenting this integration was found.

Expected behavior

A repeatable integration procedure verifies the actual outbound request against the agreed mocktioneer signing contract and reports failures clearly.

Impact

Outbound request fidelity has no documented verification against the requested signing service.

Proposed fix

Record the mocktioneer signing contract and intended adapters, then add a deterministic fixture or documented opt-in integration procedure using fictional test credentials. Coordinate with the current outbound API work before choosing the fixture entry point.

Done when

• Document the mocktioneer verification endpoint, signing rules and adapter coverage.
• Demonstrate a correctly signed outbound request and an intentionally invalid control.
• Document how to run the integration and interpret failures; ordinary unit tests require no network or provider credentials.

Related

PR #275 is related outbound work; no PR was found that fully delivers this signing check.