test: Add Accept header to bulk export polling request

The polling request in ExportOperationUtil was missing the
Accept: application/fhir+json header, causing the
ParametersToJsonInterceptor to return plain JSON instead of
the expected FHIR content type.

Author: johngrimes

server/src/test/java/au/csiro/pathling/util/ExportOperationUtil.java

@@ -316,6 +316,7 @@ public static boolean doPolling(
         webTestClient
             .get()
             .uri(pollUrl)
+            .header("Accept", "application/fhir+json")
             .exchange()
             .expectStatus()
             .is2xxSuccessful()