Subscribe a pipeline to a mailbox

Subscribes this pipeline to a connected Gmail or Outlook mailbox so the provider delivers new-message notifications directly to deliveryUrl (https). Gmail: Khotan registers the mailbox watch and a Pub/Sub push subscription whose pushes carry a Google OIDC token. Outlook: Khotan creates a Microsoft Graph subscription — Graph first validates deliveryUrl by POSTing a validationToken query parameter that the receiver must echo as text/plain within 10 seconds, and every later notification carries the clientState returned here. Secret-bearing and never cached; recover a lost clientState from the client-state endpoint.