fix(tui): avoid modifyOtherKeys for unknown tmux formats

[fcoury-oai]

Why

Codex 0.131 started enabling tmux modifyOtherKeys mode 2 when the active tmux session reported extended-keys-format csi-u, and also when that format could not be queried. The fallback was meant to help compatible tmux panes enter extended-key mode, but it breaks iTerm2 control-mode sessions on older tmux.

Issue #23711 reproduces with:

ssh -t ubuntu@192.168.68.149 'tmux -CC new -A -s main'

On tmux 3.2a, extended-keys-format is not available. With mode 2 enabled, Ctrl-C is delivered as ^[[27;5;99~ instead of the normal interrupt/control key path, so Codex does not handle it. Running with CODEX_TUI_DISABLE_KEYBOARD_ENHANCEMENT=1 restores Ctrl-C, which points at keyboard mode setup rather than chat input routing.

What Changed

• Only request modifyOtherKeys mode 2 when tmux explicitly reports extended-keys-format csi-u.
• Treat an unknown or unavailable tmux extended-key format as unsupported for this mode.
• Update the keyboard mode unit coverage so None no longer opts into modifyOtherKeys.

This preserves the explicit modern tmux csi-u path from #21943 while avoiding the unsafe fallback on older or unqueryable tmux setups.

How to Test

Regression path from #23711:

1. Start iTerm2 tmux integration against an older tmux host:

   ssh -t ubuntu@192.168.68.149 'tmux -CC new -A -s main'

2. Start patched Codex.
3. Run /keymap debug, press a regular key, then press Ctrl-C.
4. Confirm Ctrl-C closes the inspector and Codex remains responsive without CODEX_TUI_DISABLE_KEYBOARD_ENHANCEMENT=1.
5. Confirm Shift+Enter still inserts a newline in the same session.

Modern tmux compatibility path:

1. Start an ordinary tmux 3.6a server with explicit csi-u:

   tmux -L codex-csiu -f /dev/null new-session -d -s repro
   tmux -L codex-csiu set-option -g extended-keys on
   tmux -L codex-csiu set-option -g extended-keys-format csi-u
   tmux -L codex-csiu attach -t repro

2. Start patched Codex.
3. From another terminal, confirm the Codex pane reports mode=Ext 2:

   tmux -L codex-csiu list-panes -a -F '#{pane_id} mode=#{pane_key_mode} cmd=#{pane_current_command}'

4. Type one, press Shift+Enter, type two, and confirm the composer shows two lines without submitting.
5. Press Ctrl-C and confirm Codex handles it normally.

Targeted tests:

• ./tools/argument-comment-lint/run.py -p codex-tui -- --lib
• just test -p codex-tui runs the new keyboard mode test successfully; the full run currently reports two unrelated guardian feature-flag test failures:
  • app::tests::update_feature_flags_disabling_guardian_clears_manual_review_policy_without_history
  • app::tests::update_feature_flags_disabling_guardian_clears_review_policy_and_restores_default

No documentation update is needed.