Desktop edit / regenerate / restore-checkpoint send truncate_before_user_ordinal
on prompt.submit. The handler rewrote session['history'] first, then called
replace_messages, and on failure only printed to stderr and still started the
turn. When the durable write fails, in-memory history is already truncated
and history_version is bumped while state.db still holds the pre-edit tail.
The agent flush is append-only for history-dict identities, so the new
exchange is appended on top of the 'undone' turns — durable zombie history.
Fix: persist first; only then mutate memory. On failure return 5008 and
leave memory/DB unchanged.
Based on #72876 by @necoweb3. Adapted: the handler moved from
tui_gateway/server.py to tui_gateway/methods_prompt.py since the PR's base.
dsad
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