fix: Polling from exhausted stream in preprocessor (#3349)

Signed-off-by: Jacky <18255193+kthui@users.noreply.github.com>

lib/llm/src/preprocessor.rs

@@ -550,11 +550,14 @@ impl OpenAIPreprocessor {
 
                     Some((response, inner))
                 } else {
-                    // Stream has ended - check if we need to send a usage chunk
+                    // Stream has ended - must set finished to true to prevent unfold from polling
+                    // again. The stream is exhausted and will panic if polled after None.
+                    inner.finished = true;
+
+                    // Check if we need to send a usage chunk
                     if inner.response_generator.is_usage_enabled()
                         && inner.finish_reason_sent
                         && !inner.usage_chunk_sent
-                        && !inner.finished
                     {
                         inner.usage_chunk_sent = true;
 
@@ -575,7 +578,6 @@ impl OpenAIPreprocessor {
                         Some((annotated_usage, inner))
                     } else {
                         // stream closed
-                        inner.finished = true; // Mark as finished
                         None
                     }
                 }