Refactored ReadSingleEnvelopeQuery and its handler to return EnvelopeDto directly and throw NotFoundException or BadRequestException when no or multiple envelopes are found, instead of returning null. Updated imports to include custom exceptions.
Introduced ReadSingleEnvelopeQuery and its handler to enable
retrieval of a single envelope with optional user filtering.
Supports filtering by user ID, envelope ID, or UUID, includes
related documents, and maps results to EnvelopeDto using
AutoMapper for secure and flexible access.
