Queue follow-up input during user shell commands

[etraut-openai]

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

[fcoury-oai]

Replicated the issue and confirmed it is fixed in this branch.

Code looks good to me, approved!