fix: deepseek rate limit keep-alive

src/chatgpt.ts

@@ -338,6 +338,9 @@ class Chat {
         console.log("line", line);
         try {
           const jsonStr = line.slice("data:".length).trim();
+          if (jsonStr === "keep-alive") {  // for deepseek https://api-docs.deepseek.com/quick_start/rate_limit
+            continue;
+          }
           const json = JSON.parse(jsonStr) as StreamingResponseChunk;
           yield json;
         } catch (e) {