fix(gateway): mark openai role chunks unfinished

src/gateway/openai-http.ts

@@ -148,7 +148,7 @@ function writeAssistantRoleChunk(res: ServerResponse, params: { runId: string; m
     object: "chat.completion.chunk",
     created: Math.floor(Date.now() / 1000),
     model: params.model,
-    choices: [{ index: 0, delta: { role: "assistant" } }],
+    choices: [{ index: 0, delta: { role: "assistant" }, finish_reason: null }],
   });
 }