Changes to Buy credits over the Machine Payments Protocol (MPP): an agent pays with no checkout page

added the optional property docs_url to the response with the 400 status

added the optional property docs_url to the response with the 403 status

added the optional property docs_url to the response with the 503 status

added the optional property next to the response with the 400 status

added the optional property next to the response with the 403 status

added the optional property next to the response with the 503 status

added the optional property retryable to the response with the 400 status

added the optional property retryable to the response with the 403 status

added the optional property retryable to the response with the 503 status

endpoint added