Actionable Pagination Error Messages

Paginated endpoints now return a message explaining how to recover when a cursor or syncToken is expired or cannot be decoded. Previously all four cases shared a generic message. The next_cursor_expired, invalid_next_cursor, sync_token_expired, and sync_token_invalid codes are unchanged, as is the HTTP status.
The Pagination and Incremental Synchronization guide now documents cursor expiration alongside sync token expiration.