Retrieve a single SignatureRequest by ID, including signer status, completion state, and submitted response items. The id is the one an existing Requirement data-source reference carries.

This read refreshes the request's status from the e-signature provider, so it can take a provider round-trip and returns 503 when the provider is unavailable rather than serving stale values. Response-item values are visible only to the payer Account that owns the request; a payee reading its own request receives an empty array.