fix(tui): let esc exit empty shell mode

[fcoury-oai]

Summary

• exit shell mode when Esc is pressed while the absorbed ! is the only input
• add direct regression coverage plus a composer snapshot for the restored normal prompt state

Root cause

Shell mode stores the leading ! outside the editable textarea. After typing only !, the textarea is empty but the composer is still in bash mode, so the existing empty-composer Esc handling never runs.

Validation

• just fmt
• cargo test -p codex-tui bottom_pane::chat_composer::tests::esc_exits_empty_shell_mode
• cargo test -p codex-tui bottom_pane::chat_composer::tests::footer_mode_snapshots
• cargo insta pending-snapshots

cargo test -p codex-tui still reports unrelated existing /status snapshot drift in this local environment because the rendered permissions text is workspace-write with network access instead of the older read-only fixture text.

[chatgpt-codex-connector]

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