tui: return from side chat on Ctrl-D

[etraut-openai]

Why

Fixes #20264.

Side conversations are an ephemeral layer on top of the main chat. Pressing Ctrl+D from an empty side-chat composer should unwind back to the parent thread, matching the existing side-return behavior, instead of falling through to the global quit shortcut and exiting Codex.

What changed

The side-return shortcut matcher now treats Ctrl+D the same way it already treats Esc and Ctrl+C. Because app-level side-return handling runs before the chat widget's global quit handling, this returns from /side while preserving normal Ctrl+D quit behavior outside side conversations.

The existing shortcut coverage was updated to include lowercase and uppercase Ctrl+D key events.

Verification

• cargo test -p codex-tui side_return_shortcuts_match_esc_ctrl_c_and_ctrl_d
• cargo test -p codex-tui starts successfully and the new shortcut test passes, but the broader suite later aborts in the unrelated existing test app::tests::attach_live_thread_for_selection_rejects_unmaterialized_fallback_threads with a stack overflow.

[fcoury-oai]

Code looks good, ran a smoke test: started a main thread, ran /side and used Ctrl+D.

With latest alpha codex I got a complete exit and with this code I successfully exited back to the main thread.

Approved!