fix(tui): preserve Shift+Enter in tmux csi-u panes

[fcoury-oai]

Why

Inside tmux, Shift+Enter can still reach Codex as a plain Enter even when tmux has extended keys enabled. In csi-u tmux panes, Codex needs to request modifyOtherKeys mode 2 so tmux moves the pane from VT10x into extended-key mode and preserves the Shift modifier. Without that extra request, composer Shift+Enter submits the draft instead of inserting a newline.

Fixes #21699.

What Changed

• Detect tmux sessions and read the active extended-keys-format, preferring the pane-local value before falling back to the global option.
• Request modifyOtherKeys mode 2 for tmux panes using csi-u extended keys, and reset it when restoring keyboard reporting.
• Add unit coverage for tmux detection, the format gate, and the emitted modifyOtherKeys escape sequence.

How to Test

1. In tmux, configure:

   set-option -g extended-keys on
   set-option -g extended-keys-format csi-u

2. Start Codex in a fresh tmux pane from this branch.
3. From another pane, confirm the Codex pane reports mode=Ext 2:

   tmux list-panes -a -F '#{session_name}:#{window_index}.#{pane_index} mode=#{pane_key_mode} cmd=#{pane_current_command}'

4. Type a draft in the composer and press Shift+Enter; confirm it inserts a newline instead of submitting.
5. Also confirm plain Enter still submits as before.

Targeted tests:

• cargo test -p codex-tui

Notes

• Manual verification used both real Shift+Enter in iTerm2/tmux and tmux send-keys ... S-Enter to confirm the tmux pane changes from VT10x to Ext 2 and preserves newline behavior.
• On this checkout, the broader codex-tui test run currently reaches unrelated existing failures in status::tests::* plus a later stack overflow in tests::fork_last_filters_latest_session_by_cwd_unless_show_all.

[canvrno-oai]

Tested as described against the current release of codex and confirmed these changes enable shift+enter newlines in the composer when ran in tmux.