Summary: Assert at step-up that no open transaction holds a follower truncate-list entry

Problem

• __disagg_step_up has no check for active transactions (step-down has __disagg_assert_no_active_writes_callback, diagnostic only).
• __layered_build_sorted_truncates selects every entry with a transaction id, including uncommitted ones.
• An uncommitted entry is replayed onto stable (diagnostic assert on start_ts; release stamps timestamp 0), then __wt_layered_table_truncate_clear frees it while the transaction's WT_TXN_OP_FOLLOWER_TRUNCATE still points at it.

Scenario

• Follower session truncates a range and does not commit; connection reconfigures to leader.
• Release build: range deleted on stable without a commit; later commit or rollback dereferences a freed entry.

Definition of Done

• Step-up asserts (diagnostic) that no session holds an uncommitted WT_TXN_OP_FOLLOWER_TRUNCATE; release build skips uncommitted entries in the drain or fails step-up with an error.
• Python test: step-up with an open follower truncate transaction hits the assert or error instead of replaying.