Fix TUI fast mode toggle regression (#16833)

Addresses #16832

Problem: After `/fast on`, the TUI omitted an explicit service-tier
clear on later turns, so `/fast off` left app-server sessions stuck on
`priority` until restart.

Solution: Always submit the current service tier with user turns,
including an explicit clear when Fast mode is off, and add a regression
test for the `/fast on` -> `/fast off` flow.

codex-rs/tui/src/chatwidget.rs

@@ -5767,7 +5767,7 @@ impl ChatWidget {
             .personality
             .filter(|_| self.config.features.enabled(Feature::Personality))
             .filter(|_| self.current_model_supports_personality());
-        let service_tier = self.config.service_tier.map(Some);
+        let service_tier = Some(self.config.service_tier);
         let op = AppCommand::user_turn(
             items,
             self.config.cwd.to_path_buf(),

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

@@ -622,6 +622,32 @@ async fn user_turn_carries_service_tier_after_fast_toggle() {
     }
 }
 
+#[tokio::test]
+async fn user_turn_clears_service_tier_after_fast_is_turned_off() {
+    let (mut chat, mut rx, mut op_rx) = make_chatwidget_manual(Some("gpt-5.3-codex")).await;
+    chat.thread_id = Some(ThreadId::new());
+    set_chatgpt_auth(&mut chat);
+    chat.set_feature_enabled(Feature::FastMode, /*enabled*/ true);
+
+    chat.dispatch_command(SlashCommand::Fast);
+    let _events = std::iter::from_fn(|| rx.try_recv().ok()).collect::<Vec<_>>();
+
+    chat.dispatch_command_with_args(SlashCommand::Fast, "off".to_string(), Vec::new());
+    let _events = std::iter::from_fn(|| rx.try_recv().ok()).collect::<Vec<_>>();
+
+    chat.bottom_pane
+        .set_composer_text("hello".to_string(), Vec::new(), Vec::new());
+    chat.handle_key_event(KeyEvent::from(KeyCode::Enter));
+
+    match next_submit_op(&mut op_rx) {
+        Op::UserTurn {
+            service_tier: Some(None),
+            ..
+        } => {}
+        other => panic!("expected Op::UserTurn to clear service tier, got {other:?}"),
+    }
+}
+
 #[tokio::test]
 async fn compact_queues_user_messages_snapshot() {
     let (mut chat, _rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;