Allow naming forked chats from the TUI (#36036)

## What changed

- Accept an optional thread name with `/fork`, trimming surrounding whitespace.
- Apply the name to the newly forked thread and update the active session metadata.
- Keep the successful fork active and show an error when naming it fails.

## Testing

- Cover unnamed and named `/fork` dispatch, persisted names, and naming failures.

GitOrigin-RevId: 1799576f82ef0856f992a0378f041e4bab89621b

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

@@ -242,7 +242,8 @@ impl ChatWidget {
                 self.app_event_tx.send(AppEvent::OpenResumePicker);
             }
             SlashCommand::Fork => {
-                self.app_event_tx.send(AppEvent::ForkCurrentSession);
+                self.app_event_tx
+                    .send(AppEvent::ForkCurrentSession { name: None });
             }
             SlashCommand::App => {
                 let Some(thread_id) = self.thread_id else {
@@ -744,6 +745,11 @@ impl ChatWidget {
                     name: Some(trimmed.to_string()),
                 });
             }
+            SlashCommand::Fork if !trimmed.is_empty() => {
+                self.app_event_tx.send(AppEvent::ForkCurrentSession {
+                    name: Some(trimmed.to_string()),
+                });
+            }
             SlashCommand::Plan if !trimmed.is_empty() => {
                 if !self.apply_plan_slash_command() {
                     return;

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

@@ -2547,7 +2547,26 @@ async fn slash_fork_requests_current_fork() {
 
     chat.dispatch_command(SlashCommand::Fork);
 
-    assert_matches!(rx.try_recv(), Ok(AppEvent::ForkCurrentSession));
+    assert_matches!(
+        rx.try_recv(),
+        Ok(AppEvent::ForkCurrentSession { name: None })
+    );
+}
+
+#[tokio::test]
+async fn slash_fork_with_name_requests_named_fork() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.bottom_pane
+        .set_composer_text("/fork   Add User  ".to_string(), Vec::new(), Vec::new());
+
+    chat.handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+    assert_matches!(
+        rx.try_recv(),
+        Ok(AppEvent::ForkCurrentSession {
+            name: Some(name)
+        }) if name == "Add User"
+    );
 }
 
 #[tokio::test]