Change WT_ASSERT to WT_ASSERT_ALWAYS in __wt_hs_insert_updates

Summary
A data corruption bug in WT-9705 is rarely reproducible (more info in the ticket itself). In diagnostic mode, the following assert is hit.

WT_ASSERT(session, prev_upd->start_ts == WT_TS_NONE); 

However, in release mode, EBUSY is returned. It would be helpful to have a core file when/if this issue happens in MongoDB to extract more information about the bug.
 

The scope of this ticket is to convert the WT_ASSERT to WT_ASSERT_ALWAYS.