Fix TUI context window display before first TokenCount (#13896)

The TUI was showing the raw configured `model_context_window` until the
first
`TokenCount` event arrived, even though core had already emitted the
effective
runtime window on `TurnStarted`. This made the footer, status-line
context
window, and `/status` output briefly inconsistent for models/configs
where the
effective window differs from the configured value, such as the
`gpt-5.4`
1,000,000-token override reported in #13623.

Update the TUI to cache `TurnStarted.model_context_window` immediately
so
pre-token-count displays use the runtime effective window, and add
regression
coverage for the startup path.

---------

Co-authored-by: Charles Cunningham <ccunningham@openai.com>
Co-authored-by: Codex <noreply@openai.com>

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

@@ -1659,6 +1659,53 @@ async fn context_indicator_shows_used_tokens_when_window_unknown() {
     );
 }
 
+#[tokio::test]
+async fn turn_started_uses_runtime_context_window_before_first_token_count() {
+    let (mut chat, mut rx, _ops) = make_chatwidget_manual(None).await;
+
+    chat.config.model_context_window = Some(1_000_000);
+
+    chat.handle_codex_event(Event {
+        id: "turn-start".into(),
+        msg: EventMsg::TurnStarted(TurnStartedEvent {
+            turn_id: "turn-1".to_string(),
+            model_context_window: Some(950_000),
+            collaboration_mode_kind: ModeKind::Default,
+        }),
+    });
+
+    assert_eq!(
+        chat.status_line_value_for_item(&crate::bottom_pane::StatusLineItem::ContextWindowSize),
+        Some("950K window".to_string())
+    );
+    assert_eq!(chat.bottom_pane.context_window_percent(), Some(100));
+
+    chat.add_status_output();
+
+    let cells = drain_insert_history(&mut rx);
+    let context_line = cells
+        .last()
+        .expect("status output inserted")
+        .iter()
+        .map(|line| {
+            line.spans
+                .iter()
+                .map(|span| span.content.as_ref())
+                .collect::<String>()
+        })
+        .find(|line| line.contains("Context window"))
+        .expect("context window line");
+
+    assert!(
+        context_line.contains("950K"),
+        "expected /status to use TurnStarted context window, got: {context_line}"
+    );
+    assert!(
+        !context_line.contains("1M"),
+        "expected /status to avoid raw config context window, got: {context_line}"
+    );
+}
+
 #[cfg_attr(
     target_os = "macos",
     ignore = "system configuration APIs are blocked under macOS seatbelt"
@@ -1952,7 +1999,7 @@ fn lines_to_single_string(lines: &[ratatui::text::Line<'static>]) -> String {
 }
 
 fn status_line_text(chat: &ChatWidget) -> Option<String> {
-    chat.bottom_pane.status_line_text()
+    chat.status_line_text()
 }
 
 fn make_token_info(total_tokens: i64, context_window: i64) -> TokenUsageInfo {