feat(tui): allow function keys through f24 in keymaps

[fcoury-oai]

Why

Closes #25006.

tui.keymap currently rejects F13 even though Codex's terminal event layer can report higher function keys. This prevents users from using common remappings such as Caps Lock to F13.

What Changed

• Define a shared portable upper bound of F24 for stored TUI keybindings.
• Accept f13 through f24 in config normalization and runtime parsing.
• Allow /keymap capture to persist F13 through F24.
• Update the unsupported-function-key error and add boundary tests for F13, F24, and F25.

How to Test

1. Add a binding such as:

   [tui.keymap.global]
   open_transcript = "f13"

2. Start Codex and press the remapped F13 key.

3. Confirm Codex loads the config without the previous F1 through F12 error and the action runs.

4. Open /keymap, capture F13 for an action, and confirm the saved binding is f13.

5. As a regression check, try to capture F25 and confirm Codex reports that only F1 through F24 can be stored.

Targeted tests:

• just test -p codex-config
• just test -p codex-tui function_keys

Full just test -p codex-tui completed with 2,752 passing tests, 4 skipped tests, and two unrelated guardian feature-flag failures:

• app::tests::update_feature_flags_disabling_guardian_clears_review_policy_and_restores_default
• app::tests::update_feature_flags_disabling_guardian_clears_manual_review_policy_without_history

[etraut-openai]

Code looks good.

I tested locally and confirmed that it worked with terminals that support these extended function keys and that there aren't any issues with terminals that don't support them.