Create a callback endpoint

Creates a reusable callback endpoint in the caller's project and environment. The destination must be HTTPS on effective port 443, contain no URL credentials or fragment, and resolve only to public unicast addresses. The endpoint is active and usable the moment it exists; there is nothing to verify. A disabled endpoint is never delivered to, and a schedule write naming one answers not_found, because disabling is terminal rather than a state to recover from. The per-endpoint secret is returned to this call and to no other call an API credential or an OAuth token can make; a signed-in member can read it again through endpoints.revealSigningSecret. There is no endpoint-count ceiling. Disabled endpoints remain readable for audit; request and DNS inspection admission bound creation work and reads are paged. Optional headers set reusable defaults. Ordinary values remain readable; secret: true encrypts a write-only value that is never returned. Header edits use explicit set, keep, and remove actions.