Fix group on configured tasks

Fixes #129686

src/vs/workbench/contrib/tasks/browser/abstractTaskService.ts

@@ -1223,7 +1223,7 @@ export abstract class AbstractTaskService extends Disposable implements ITaskSer
 				toCustomize.problemMatcher = task.configurationProperties.problemMatchers;
 			}
 			if (task.configurationProperties.group) {
-				toCustomize.group = task.configurationProperties.group;
+				toCustomize.group = TaskConfig.GroupKind.to(task.configurationProperties.group);
 			}
 		}
 		if (!toCustomize) {

src/vs/workbench/contrib/tasks/common/taskConfiguration.ts

@@ -1230,7 +1230,7 @@ const partialSource: Partial<Tasks.TaskSource> = {
 	config: undefined
 };
 
-namespace GroupKind {
+export namespace GroupKind {
 	export function from(this: void, external: string | GroupKind | undefined): Tasks.TaskGroup | undefined {
 		if (external === undefined) {
 			return undefined;
@@ -1244,6 +1244,18 @@ namespace GroupKind {
 		}
 		return undefined;
 	}
+
+	export function to(group: Tasks.TaskGroup | string): GroupKind | string {
+		if (Types.isString(group)) {
+			return group;
+		} else if (!group.isDefault) {
+			return group._id;
+		}
+		return {
+			kind: group._id,
+			isDefault: group.isDefault
+		};
+	}
 }
 
 namespace TaskDependency {