Adds --once to /model across CLI, TUI, and gateway: switch model for the
next turn only, restoring the previous model in a finally block so
success, exception, and interrupt all revert. Parsing extends
parse_model_flags_detailed(); resolve_persist_behavior() treats --once
as a persistence opt-out; --global + --once is rejected.
Salvaged from PR #29923 (image-generation lane split to #59815 per
review; conflict resolution against current main by the maintainers).
