♻️ Extract private method to reduce duplication (#1995)

Author: KATO-Hiro

src/lib/clients/aizu_online_judge.ts

@@ -568,8 +568,7 @@ export class AojChallengesApiClient extends AojTasksApiClientBase<ChallengeParam
       cacheKey: this.getCacheKey(contestType, round),
       endpoint: this.buildEndpoint(['challenges', 'cl', contestType, round]),
       errorMessage: `Failed to fetch ${this.getContestTypeLabel(contestType)} ${round} contests from AOJ API`,
-      validateResponse: (data) =>
-        'contests' in data && Array.isArray(data.contests) && data.contests.length > 0,
+      validateResponse: (data) => this.validateApiResponse(data),
       transformer: (data) => this.transformToContests(data),
       label: `${this.getContestTypeLabel(contestType)} ${round}`,
     });
@@ -582,8 +581,7 @@ export class AojChallengesApiClient extends AojTasksApiClientBase<ChallengeParam
       cacheKey: this.getCacheKey(contestType, round),
       endpoint: this.buildEndpoint(['challenges', 'cl', contestType, round]),
       errorMessage: `Failed to fetch ${this.getContestTypeLabel(contestType)} ${round} tasks from AOJ API`,
-      validateResponse: (data) =>
-        'contests' in data && Array.isArray(data.contests) && data.contests.length > 0,
+      validateResponse: (data) => this.validateApiResponse(data),
       transformer: (data) => this.transformToTasks(data),
       label: `${this.getContestTypeLabel(contestType)} ${round}`,
     });
@@ -604,6 +602,10 @@ export class AojChallengesApiClient extends AojTasksApiClientBase<ChallengeParam
     return `aoj_${contestType.toLowerCase()}_${round.toLowerCase()}`;
   }
 
+  private validateApiResponse(data: AOJChallengeContestAPI): boolean {
+    return 'contests' in data && Array.isArray(data.contests) && data.contests.length > 0;
+  }
+
   private transformToContests(data: AOJChallengeContestAPI): ContestsForImport {
     return data.contests.flatMap((contest: ChallengeContest) =>
       contest.days