Nicer highlighting of slash commands, /plan accepts prompt args and pasted images (#10269)

## Summary
- Make typed slash commands become text elements when the user hits
space, including paste‑burst spaces.
- Enable `/plan` to accept inline args and submit them in plan mode,
mirroring `/review` behavior and blocking submission while a task is
running.
- Preserve text elements/attachments for slash commands that take args.

<img width="1510" height="500" alt="image"
src="https://github.com/user-attachments/assets/446024df-b69a-4249-85db-1a85110e07f1"
/>

## Changes
- Add safe helper to insert element ranges in the textarea.
- Extend command‑with‑args pipeline to carry text elements and reuse
submission prep.
- Update `/plan` dispatch to switch to plan mode then submit prompt +
elements.
- Document new composer behavior and add tests.

## Notes
- `/plan` is blocked during active tasks (same as `/review`).
- Slash‑command elementization recognizes built‑ins and `/prompts:`
custom commands only.

## Codex author
`codex fork 019c16d3-4520-7bb0-9b9d-48720d40a8ab`

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

@@ -2316,6 +2316,50 @@ async fn plan_slash_command_switches_to_plan_mode() {
     assert_eq!(chat.current_collaboration_mode(), &initial);
 }
 
+#[tokio::test]
+async fn plan_slash_command_with_args_submits_prompt_in_plan_mode() {
+    let (mut chat, _rx, mut op_rx) = make_chatwidget_manual(None).await;
+    chat.set_feature_enabled(Feature::CollaborationModes, true);
+
+    let configured = codex_core::protocol::SessionConfiguredEvent {
+        session_id: ThreadId::new(),
+        forked_from_id: None,
+        thread_name: None,
+        model: "test-model".to_string(),
+        model_provider_id: "test-provider".to_string(),
+        approval_policy: AskForApproval::Never,
+        sandbox_policy: SandboxPolicy::ReadOnly,
+        cwd: PathBuf::from("/home/user/project"),
+        reasoning_effort: Some(ReasoningEffortConfig::default()),
+        history_log_id: 0,
+        history_entry_count: 0,
+        initial_messages: None,
+        rollout_path: None,
+    };
+    chat.handle_codex_event(Event {
+        id: "configured".into(),
+        msg: EventMsg::SessionConfigured(configured),
+    });
+
+    chat.bottom_pane
+        .set_composer_text("/plan build the plan".to_string(), Vec::new(), Vec::new());
+    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
+
+    let items = match next_submit_op(&mut op_rx) {
+        Op::UserTurn { items, .. } => items,
+        other => panic!("expected Op::UserTurn, got {other:?}"),
+    };
+    assert_eq!(items.len(), 1);
+    assert_eq!(
+        items[0],
+        UserInput::Text {
+            text: "build the plan".to_string(),
+            text_elements: Vec::new(),
+        }
+    );
+    assert_eq!(chat.active_collaboration_mode_kind(), ModeKind::Plan);
+}
+
 #[tokio::test]
 async fn collaboration_modes_defaults_to_code_on_startup() {
     let codex_home = tempdir().expect("tempdir");