tui: add feature-gated /plan slash command to switch to Plan mode (#10103)

## Summary
Adds a simple `/plan` slash command in the TUI that switches the active
collaboration mode to Plan mode. The command is only available when the
`collaboration_modes` feature is enabled.

## Changes
- Add `plan_mask` helper in `codex-rs/tui/src/collaboration_modes.rs`
- Add `SlashCommand::Plan` metadata in
`codex-rs/tui/src/slash_command.rs`
- Implement and hard-gate `/plan` dispatch in
`codex-rs/tui/src/chatwidget.rs`
- Hide `/plan` when collaboration modes are disabled in
`codex-rs/tui/src/bottom_pane/slash_commands.rs`
- Update command popup tests in
`codex-rs/tui/src/bottom_pane/command_popup.rs`
- Add a focused unit test for `/plan` in
`codex-rs/tui/src/chatwidget/tests.rs`

## Behavior notes
- `/plan` is now a no-op if `Feature::CollaborationModes` is disabled.
- When enabled, `/plan` switches directly to Plan mode without opening
the picker.

## Codex author
`codex resume 019c05da-d7c3-7322-ae2c-3ca38d0ef702`

codex-rs/tui/src/chatwidget/tests.rs

@@ -2267,6 +2267,19 @@ async fn collab_slash_command_opens_picker_and_updates_mode() {
     }
 }
 
+#[tokio::test]
+async fn plan_slash_command_switches_to_plan_mode() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
+    chat.set_feature_enabled(Feature::CollaborationModes, true);
+    let initial = chat.current_collaboration_mode().clone();
+
+    chat.dispatch_command(SlashCommand::Plan);
+
+    assert!(rx.try_recv().is_err(), "plan should not emit an app event");
+    assert_eq!(chat.active_collaboration_mode_kind(), ModeKind::Plan);
+    assert_eq!(chat.current_collaboration_mode(), &initial);
+}
+
 #[tokio::test]
 async fn collaboration_modes_defaults_to_code_on_startup() {
     let codex_home = tempdir().expect("tempdir");