Avoid background_thread variable usage in ha_innobase::open()

The current implementation of ha_innobase::open relies on a thread-local variable (background_thread) to determine if a table is being opened for background virtual column purging. This approach is brittle and potentially problematic . We should replace this with a dedicated handler flag, HA_OPEN_FOR_INNODB_PURGE, to explicitly signal when metadata initialization (like auto-increment) should be skipped.

The reliance on THDVAR(thd, background_thread) inside ha_innobase::open is a side-effect-heavy way to detect the purge thread.