fix(mcp): port upstream unregister schema updates (id/url union) (#1953)

based on some feedback in upstream backstage while porting the register/unregister
function there, the usefule change of allowing unregister to take either the location ID
(which unless you save it when registering the entities, is hard to find) or the location URL used to register the location

Assisted-by: Claude CLI

Author: gabemontero