/update calls dieWithCode(42) which tears down the gateway and
hard-exits the Node process — the same PTY-killing path that /exit
and /quit use. In the hosted dashboard chat there is no Python
update wrapper to catch exit code 42, and the PTY death bricks the
tab until a browser refresh.
Mirror the DASHBOARD_TUI_MODE guard that #48882 added for /exit and
/quit: refuse early with an explanatory message.
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