importinto: integrate table mode for NextGen kernel

## Enhancement

### Background

`IMPORT INTO` in the classic kernel already integrates table mode through #61199. The NextGen kernel should provide the same protection for `IMPORT INTO`.

When a NextGen `IMPORT INTO` job starts, the target table should enter `Import` mode. While the table is in `Import` mode, users should not be able to write data to the target table or run DDL that changes the target table. When the `IMPORT INTO` job finishes, fails, or is canceled, the table mode should be changed back to `Normal`.

This prevents users from accidentally modifying the target table during a long-running import, especially when the dataset is large.