parameter silently ignored (even for nonexistent supplement URLs)

[jmandel]

Bug

The useSupplement parameter is accepted but has no effect. Even a completely nonexistent supplement URL produces no error and no change in output.

Repro

# $lookup with nonexistent supplement — no error
curl -s -o /dev/null -w '%{http_code}' \
  'https://tx-dev.fhir.org/r5/CodeSystem/$lookup?system=http://loinc.org&code=8480-6&useSupplement=http://example.org/nonexistent' \
  -H 'Accept: application/fhir+json'
# Returns: 200 (should error — supplement doesn't exist)

# $expand with nonexistent supplement — no error
curl -s -o /dev/null -w '%{http_code}' \
  'https://tx-dev.fhir.org/r5/ValueSet/$expand?url=http://hl7.org/fhir/ValueSet/administrative-gender&useSupplement=http://example.org/fake' \
  -H 'Accept: application/fhir+json'
# Returns: 200 (should error)

Spec reference

R5: useSupplement (IN, 0..*, canonical) — "The supplement must be used when performing an expansion." Unknown supplements should produce an error.