Initiate and poll a FHIR bulk data export ($export) to retrieve patient data as NDJSON files

Steps

1. Initiate a system-level export: GET [base]/$export with headers Accept: application/fhir+json and Prefer: respond-async; optionally add _type=[ResourceType1,ResourceType2] to limit exported resource types.
2. The server responds with HTTP 202 Accepted and a Content-Location header containing a polling URL for the export job status.
3. Poll the Content-Location URL; while the job is running the server returns 202 with an optional X-Progress header; when complete it returns 200 with a JSON body containing output[] array.
4. Each output[] element has type (FHIR resource type) and url (URL to download an NDJSON file); each line in the NDJSON file is a complete FHIR resource JSON object.
5. Download each NDJSON file with the same bearer token; process line by line rather than loading entire files into memory, as files may contain millions of records.
6. Check the error[] array in the completed export manifest for any resource types that failed to export; handle partial export failures gracefully.