http: fix drain event with cork/uncork in ServerResponse

When using cork() and uncork() with ServerResponse, the drain event
was not reliably emitted after uncorking. This occurred because the
uncork() method did not check if a drain was pending (kNeedDrain flag)
after flushing the chunked buffer.

This fix ensures that when uncork() successfully flushes buffered data
and a drain was needed, the drain event is emitted immediately.

Fixes: #60432

Author: ThierryMT

lib/_http_outgoing.js

@@ -287,14 +287,20 @@ OutgoingMessage.prototype.uncork = function uncork() {
       callbacks.push(buf[n + 2]);
     }
   }
-  this._send(crlf_buf, null, callbacks.length ? (err) => {
+  const ret = this._send(crlf_buf, null, callbacks.length ? (err) => {
     for (const callback of callbacks) {
       callback(err);
     }
   } : null);
 
   this[kChunkedBuffer].length = 0;
   this[kChunkedLength] = 0;
+
+  // If we successfully flushed and had pending drain, emit it
+  if (ret && this[kNeedDrain]) {
+    this[kNeedDrain] = false;
+    this.emit('drain');
+  }
 };
 
 OutgoingMessage.prototype.setTimeout = function setTimeout(msecs, callback) {