Summary: Truncate-list conflict checks run before the isolation check and return WT_ROLLBACK instead of ENOTSUP

Problem

• Writes at read-committed or read-uncommitted isolation are rejected with ENOTSUP by __wt_txn_id_check on all tables.
• On a follower, __wt_layered_table_truncate_detect_write_conflict runs first and has no isolation test.
• Read-committed writer inside an uncommitted truncate range: WT_ROLLBACK (retryable) instead of ENOTSUP (usage error).
• Read-uncommitted writer: every entry counts as visible, no list conflict, then ENOTSUP.

Definition of Done

• List checks skip or defer to the isolation check so the error code matches the leader and per-key remove.
• Python test for both isolation levels on a follower.