fix: correctly retrieve operation from request body

Author: koenpunt

spring-graphql/src/main/java/org/springframework/graphql/server/WebGraphQlRequest.java

@@ -105,9 +105,9 @@ private static String getQuery(Map<String, Object> body) {
 
 	@Nullable
 	private static String getOperation(Map<String, Object> body) {
-		Object value = body.get("operation");
+		Object value = body.get("operationName");
 		if (value != null && !(value instanceof String)) {
-			throw new ServerWebInputException("Invalid value for 'operation'");
+			throw new ServerWebInputException("Invalid value for 'operationName'");
 		}
 		return (String) value;
 	}

spring-graphql/src/test/java/org/springframework/graphql/server/WebGraphQlRequestTests.java

@@ -39,7 +39,7 @@ public class WebGraphQlRequestTests {
 	void invalidBody() {
 		testInvalidBody(Map.of());
 		testInvalidBody(Map.of("query", Collections.emptyMap()));
-		testInvalidBody(Map.of("query", "query { foo }", "operation", Collections.emptyMap()));
+		testInvalidBody(Map.of("query", "query { foo }", "operationName", Collections.emptyMap()));
 		testInvalidBody(Map.of("query", "query { foo }", "variables", "not-a-map"));
 		testInvalidBody(Map.of("query", "query { foo }", "extensions", "not-a-map"));
 	}