Fixes#17954.
## Why
When a manual shell command like `!sleep 10` is running, submitting
plain text such as `hi` currently sends that text as a steer for the
active shell turn. User shell turns are not steerable like model turns,
so the TUI can remain stuck in `Working` after the shell command
finishes.
## What Changed
- Detect when the only active work is one or more
`ExecCommandSource::UserShell` commands.
- Queue plain submitted input in that state so it drains after the shell
command and shell turn complete.
- Preserve `!cmd` submissions during running work so explicit shell
commands keep their existing behavior.
- Add regression coverage for the `!sleep 10` plus `hi` flow in
`chatwidget::tests::exec_flow::user_message_during_user_shell_command_is_queued_not_steered`.
## Verification
- Manually confirmed hang before the fix and no hang after the fix
Eric TrautandGitHub
b7fec54354