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`

codex-rs/tui/src/keymap_setup.rs

@@ -33,6 +33,7 @@ pub(crate) use picker::build_keymap_picker_params_with_filter;
 
 use codex_config::types::KeybindingSpec;
 use codex_config::types::KeybindingsSpec;
+use codex_config::types::MAX_FUNCTION_KEY;
 use codex_config::types::TuiKeymap;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
@@ -737,11 +738,11 @@ fn key_parts_to_config_key_spec(
         KeyCode::End => "end".to_string(),
         KeyCode::PageUp => "page-up".to_string(),
         KeyCode::PageDown => "page-down".to_string(),
-        KeyCode::F(number) if (1..=12).contains(&number) => format!("f{number}"),
+        KeyCode::F(number) if (1..=MAX_FUNCTION_KEY).contains(&number) => format!("f{number}"),
         KeyCode::F(_) => {
-            return Err(
-                "Only function keys F1 through F12 can be stored in `tui.keymap`.".to_string(),
-            );
+            return Err(format!(
+                "Only function keys F1 through F{MAX_FUNCTION_KEY} can be stored in `tui.keymap`."
+            ));
         }
         KeyCode::Char(' ') => "space".to_string(),
         KeyCode::Char(mut ch) => {
@@ -1610,6 +1611,22 @@ mod tests {
         );
     }
 
+    #[test]
+    fn key_capture_serializes_function_keys_through_f24() {
+        assert_eq!(
+            key_event_to_config_key_spec(KeyEvent::from(KeyCode::F(13))),
+            Ok("f13".to_string())
+        );
+        assert_eq!(
+            key_event_to_config_key_spec(KeyEvent::from(KeyCode::F(24))),
+            Ok("f24".to_string())
+        );
+        assert_eq!(
+            key_event_to_config_key_spec(KeyEvent::from(KeyCode::F(25))),
+            Err("Only function keys F1 through F24 can be stored in `tui.keymap`.".to_string())
+        );
+    }
+
     #[test]
     fn key_capture_serializes_c0_control_chars_as_ctrl_bindings() {
         assert_eq!(