Rename success response key from 'error' to 'data' in admin errors API endpoints

[coderabbitai]

Summary

The GET /api/admin/errors/:id and PATCH /api/admin/errors/:id handlers in server/api/src/routes/admin/errors.ts currently return success payloads using the key error (e.g., { error: row }), which is misleading since error conventionally indicates an error message.

This was intentionally deferred from PR #905 (comment: #905 (comment)) because the admin client (admin/src/api/admin.ts) is already typed against the { error: ApiErrorRow } shape.

Acceptance Criteria

• Rename the success response key in both handlers (GET /:id and PATCH /:id) from error to a more descriptive name such as data or apiError.
• Update the corresponding admin client types and fetch functions in admin/src/api/admin.ts in the same PR.
• Ensure tests in server/api/src/__tests__/routes/admin/errors.test.ts and admin/src/api/admin.test.ts are updated accordingly.

References

• PR Release: develop を main にマージ #905: Release: develop を main にマージ #905
• Review comment: Release: develop を main にマージ #905 (comment)
• Requester: @otomatty