fix: use hasContentLen instead of hasContentLength for header removal

Use the post-headers flag (hasContentLen) that reflects the effective
headers after writeHead() obj headers are applied, rather than the
initial hasContentLength value.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/js/node/_http_server.ts

@@ -1187,7 +1187,8 @@ function _writeHead(statusCode, reason, obj, response) {
       // Trailers cannot be used without chunked encoding
       if (response.hasHeader("trailer")) {
         // If "Trailer" header is explicitly set, that's an error
-        if (hasContentLength) {
+        // Use hasContentLen (not hasContentLength) to reflect current headers after obj applied
+        if (hasContentLen) {
           response.removeHeader("trailer");
         } else {
           response.removeHeader("content-length");