Surface context window error to the client (#4675)

In the past, we were treating `input exceeded context window` as a
streaming error and retrying on it. Retrying on it has no point because
it won't change the behavior. In this PR, we surface the error to the
client without retry and also send a token count event to indicate that
the context window is full.

<img width="650" height="125" alt="image"
src="https://github.com/user-attachments/assets/c26b1213-4c27-4bfc-90f4-51a270a3efd5"
/>

codex-rs/core/src/codex.rs

@@ -782,6 +782,17 @@ impl Session {
         self.send_event(event).await;
     }
 
+    async fn set_total_tokens_full(&self, sub_id: &str, turn_context: &TurnContext) {
+        let context_window = turn_context.client.get_model_context_window();
+        if let Some(context_window) = context_window {
+            {
+                let mut state = self.state.lock().await;
+                state.set_token_usage_full(context_window);
+            }
+            self.send_token_count_event(sub_id).await;
+        }
+    }
+
     /// Record a user input item to conversation history and also persist a
     /// corresponding UserMessage EventMsg to rollout.
     async fn record_input_and_rollout_usermsg(&self, response_input: &ResponseInputItem) {
@@ -1938,6 +1949,10 @@ async fn run_turn(
             Err(CodexErr::Interrupted) => return Err(CodexErr::Interrupted),
             Err(CodexErr::EnvVar(var)) => return Err(CodexErr::EnvVar(var)),
             Err(e @ CodexErr::Fatal(_)) => return Err(e),
+            Err(e @ CodexErr::ContextWindowExceeded) => {
+                sess.set_total_tokens_full(&sub_id, turn_context).await;
+                return Err(e);
+            }
             Err(CodexErr::UsageLimitReached(e)) => {
                 let rate_limits = e.rate_limits.clone();
                 if let Some(rate_limits) = rate_limits {