feat(tui): allow function keys through f24 in keymaps (#25329)

## 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:

   ```toml
   [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`