NRL-1215 Fix error code for duplicates

Author: axelkrastek1-nhs

layer/nrlf/core/request.py

@@ -109,7 +109,7 @@ def raise_when_duplicate_keys(json_content: str) -> None:
     if duplicates:
         raise OperationOutcomeError(
             severity="error",
-            code="required",
+            code="invalid",
             details=SpineErrorConcept.from_code("MESSAGE_NOT_WELL_FORMED"),
             diagnostics=f"Duplicate keys found in FHIR document: {duplicates}",
             expression=paths,

layer/nrlf/core/tests/test_request.py

@@ -170,7 +170,7 @@ def test_parse_body_valid_docref_with_duplicate_key():
         "issue": [
             {
                 "severity": "error",
-                "code": "required",
+                "code": "invalid",
                 "details": {
                     "coding": [
                         {