Preview an archive-vendor connector's readiness from draft form fields
Runs the same read-only preflight as GET /archive-connectors/:vendor/readiness — transport compatibility, delivery-endpoint grammar, record classes, and whether a credential reference is set — against DRAFT register-form fields that have not been saved yet, so an operator can run ‘Check readiness’ (a configuration check, not a live connectivity test) while configuring a new connector, before committing to PUT /archive-connectors/:vendor. Nothing is persisted; no network call is made and no credential material is read or stored. Live reachability, authentication, and a sample delivery are exercised by the delivery worker once the connector is active.