📚 Add validation to helper method (#2627)

Author: KATO-Hiro

docs/dev-notes/2025-09-23/contest-task-pair-mapping/plan.md

@@ -361,7 +361,22 @@ export class TaskMapAdapter<T> {
   // 他のメソッド (has, set, delete) も同様に実装
 }
 
+/**
+ * Creates a unique key for a ContestTaskPair using contestId and taskId.
+ * Throws an error if either argument is an empty string.
+ *
+ * @param contestId - The ID of the contest.
+ * @param taskId - The ID of the task.
+ * @returns A string in the format "contestId:taskId".
+ * @throws Will throw an error if contestId or taskId is empty.
+ */
 export function createContestTaskPairKey(contestId: string, taskId: string): ContestTaskPairKey {
+  if (!contestId || contestId.trim() === '') {
+    throw new Error('contestId must be a non-empty string');
+  }
+  if (!taskId || taskId.trim() === '') {
+    throw new Error('taskId must be a non-empty string');
+
   return `${contestId}:${taskId}`;
 }
 ```
@@ -569,6 +584,8 @@ describe('createContestTaskPairKey', () => {
   test('expect to validate input parameters', () => {
     expect(() => createContestTaskPairKey('', 'task')).toThrow();
     expect(() => createContestTaskPairKey('contest', '')).toThrow();
+    expect(() => createContestTaskPairKey(' ', 'task')).toThrow();
+    expect(() => createContestTaskPairKey('contest', ' ')).toThrow();
   });
 });
 ```