R5 reverse translation (/) unimplemented

[jmandel]

Bug

R5 added reverse translation parameters (targetCode, targetSystem, targetCoding, targetCodeableConcept) to look up source concepts that map to a given target. The server rejects all of these, demanding source-direction parameters instead.

Repro

curl -s 'https://tx-dev.fhir.org/r5/ConceptMap/$translate?targetCode=M&targetSystem=http://terminology.hl7.org/CodeSystem/v3-AdministrativeGender' \
  -H 'Accept: application/fhir+json'
# Returns 400: "Must provide sourceCode (with system), sourceCoding, or sourceCodeableConcept"

Expected: Either a successful reverse translation result, or HTTP 501 with OperationOutcome stating that reverse translation is not supported.

Spec reference

R5 section 6.15.22: the server SHALL accept one of sourceCode+sourceSystem, sourceCoding, sourceCodeableConcept, targetCode+targetSystem, targetCoding, or targetCodeableConcept.