Fix upstream request line handling

Program.cs

@@ -378,7 +378,7 @@ class Program
     {
         var lines = req.Raw.Split("\r\n", StringSplitOptions.None);
         var sb = new StringBuilder();
-        sb.Append(lines[0]).Append("\r\n");
+        sb.Append(BuildUpstreamRequestLine(req, lines[0])).Append("\r\n");
 
         for (int i = 1; i < lines.Length; i++)
         {
@@ -410,6 +410,26 @@ class Program
         return result;
     }
 
+    static string BuildUpstreamRequestLine(HttpRequest req, string originalLine)
+    {
+        if (string.IsNullOrWhiteSpace(originalLine))
+            return originalLine ?? string.Empty;
+
+        var parts = originalLine.Split(' ', StringSplitOptions.RemoveEmptyEntries);
+        if (parts.Length < 3)
+            return originalLine;
+
+        string method = parts[0];
+        string version = parts[2];
+        string url = req.Url;
+
+        if (string.IsNullOrWhiteSpace(url))
+            return originalLine;
+
+        // Upstream forward proxy expects absolute-form: METHOD http://host/path HTTP/1.1
+        return $"{method} {url} {version}";
+    }
+
     static string BuildUpstreamProxyAuthHeader()
     {
         if (string.IsNullOrWhiteSpace(UPSTREAM_USER))