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.rs

@@ -1711,6 +1711,27 @@ impl ChatWidget {
         }
     }
 
+    fn apply_turn_started_context_window(&mut self, model_context_window: Option<i64>) {
+        let info = match self.token_info.take() {
+            Some(mut info) => {
+                info.model_context_window = model_context_window;
+                info
+            }
+            None => {
+                let Some(model_context_window) = model_context_window else {
+                    return;
+                };
+                TokenUsageInfo {
+                    total_token_usage: TokenUsage::default(),
+                    last_token_usage: TokenUsage::default(),
+                    model_context_window: Some(model_context_window),
+                }
+            }
+        };
+
+        self.apply_token_info(info);
+    }
+
     fn apply_token_info(&mut self, info: TokenUsageInfo) {
         let percent = self.context_remaining_percent(&info);
         let used_tokens = self.context_used_tokens(&info, percent.is_some());
@@ -4736,8 +4757,9 @@ impl ChatWidget {
                 self.on_agent_reasoning_final();
             }
             EventMsg::AgentReasoningSectionBreak(_) => self.on_reasoning_section_break(),
-            EventMsg::TurnStarted(_) => {
+            EventMsg::TurnStarted(event) => {
                 if !is_resume_initial_replay {
+                    self.apply_turn_started_context_window(event.model_context_window);
                     self.on_task_started();
                 }
             }
@@ -8440,6 +8462,11 @@ impl ChatWidget {
         &self.config
     }
 
+    #[cfg(test)]
+    pub(crate) fn status_line_text(&self) -> Option<String> {
+        self.bottom_pane.status_line_text()
+    }
+
     pub(crate) fn clear_token_usage(&mut self) {
         self.token_info = None;
     }