認証エンドポイントの修正とリファクタリング

[SasakiTakatsugu]

変更内容

• refreshエンドポイントのバグ修正（連想配列とFETCH_RESPONSEを使用）
• credentialsエンドポイントをCallbackApiControllerからIndexControllerに移行
• SDK標準コントローラーへの依存を削除

[Copilot]

Pull Request Overview

This PR refactors authentication credential handling by replacing the third-party CallbackApiController with a custom implementation in IndexController. The changes consolidate credential retrieval logic within the application's own controller and fix the refresh token authentication flow.

Key changes:

• Removed dependency on CallbackApiController and implemented custom credentials() method
• Fixed refresh() method to use correct API parameter format
• Standardized both methods to use the same API call pattern with proper response handling

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
routes/api.php	Updated /credentials route to use IndexController::credentials instead of external CallbackApiController
app/Http/Controllers/IndexController.php	Added credentials() method and refactored refresh() method with corrected parameter structure and consistent error logging

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