Handle the 202 Accepted / Content-Location / X-Progress async pattern for long-running FHIR operations

Steps

1. Send the initial operation request (e.g., $export, $import, or $everything) with the Prefer: respond-async header to signal that the client accepts an asynchronous response
2. On receiving a 202 Accepted response, read the Content-Location header — this is the status polling URL; also read the optional Retry-After header which specifies the minimum polling interval in seconds
3. Implement a polling loop that sends GET requests to the Content-Location URL at intervals no shorter than the Retry-After value; read X-Progress headers on 202 responses for informational status messages
4. On receiving a 200 OK from the status endpoint, parse the completed response body as defined by the operation specification
5. Handle terminal error states: a 4xx or 5xx on the status URL indicates the operation failed; parse the OperationOutcome in the body to extract the error details and determine whether the operation is retryable