ACP's save_session() did a non-atomic clear_messages() + append_message()
loop. If any message hit an exception mid-loop (bad tool_call shape, etc.),
the DELETE had already committed and the persisted conversation was lost.
SessionDB.replace_messages() wraps DELETE + bulk INSERT in a single
BEGIN IMMEDIATE transaction that rolls back on any exception, so a bad
message can no longer clobber previously-persisted history.
Salvages @Awsh1's PR #13675 — uses the existing replace_messages()
helper (which covers more message fields than the PR's own copy)
instead of adding a duplicate.
5795b3be4e