OZG-6867 Throw exception instead of logging a warning

src/main/java/de/ozgcloud/admin/keycloak/GroupMapper.java

@@ -44,7 +44,7 @@ abstract class GroupMapper {
 			return null;
 		}
 		if (values.size() > 1 && values.stream().distinct().count() > 1) {
-			LOG.warn("Group contains multiple values for {}. The first one is taken.", keycloakApiProperties.getOrganisationsEinheitIdKey());
+			throw new GroupRepresentationMappingException("Group contains multiple values for organisationsEinheitId: %s".formatted(values));
 		}
 		return values.getFirst();
 	}

src/main/java/de/ozgcloud/admin/keycloak/GroupRepresentationMappingException.java

+package de.ozgcloud.admin.keycloak;
+
+public class GroupRepresentationMappingException extends RuntimeException {
+
+	public GroupRepresentationMappingException(String message) {
+		super(message);
+	}
+}

src/test/java/de/ozgcloud/admin/keycloak/GroupMapperTest.java

@@ -235,14 +235,14 @@ class GroupMapperTest {
 		}
 
 		@Test
-		void shouldReturnFirstValueIfMultipleAreAvailable() {
+		void shouldThrowExceptionIfMultipleValuesAreAvailable() {
 			givenOrganisationsEinheitIdProperty();
 			var value = GroupRepresentationTestFactory.ORGANISATIONS_EINHEIT_ID;
 			var value2 = UUID.randomUUID().toString();
 
-			var result = mapper.getOrganisationsEinheitId(Map.of(ORGANIZATIONS_EINHEIT_ID_ATTRIBUTE, List.of(value, value2)));
-
-			assertThat(result).isEqualTo(value);
+			assertThatExceptionOfType(GroupRepresentationMappingException.class)
+					.isThrownBy(() ->  mapper.getOrganisationsEinheitId(Map.of(ORGANIZATIONS_EINHEIT_ID_ATTRIBUTE, List.of(value, value2))))
+					.withMessage("Group contains multiple values for organisationsEinheitId: %s", List.of(value, value2));
 		}
 	}