server: add data: [DONE] to /chat/completions stream response (#9459)

examples/server/server.cpp

@@ -2993,6 +2993,8 @@ int main(int argc, char ** argv) {
                 }, [&](json error_data) {
                     server_sent_event(sink, "error", error_data);
                 });
+                static const std::string ev_done = "data: [DONE]\n\n";
+                sink.write(ev_done.data(), ev_done.size());
                 sink.done();
                 return true;
             };

examples/server/tests/features/steps/steps.py

@@ -1020,6 +1020,8 @@ async def oai_chat_completions(user_prompt,
                             event_data = line.split(': ', 1)
                             assert event_data[0] == 'data', f'Bad event code received: ```{event_data}```'
                             chunk_raw = event_data[1]
+                            if chunk_raw == '[DONE]':
+                                break
 
                             chunk = json.loads(chunk_raw)
                             assert len(chunk['choices']) == 1, f"no choices provided, line ```{line}```"