Implement request smuggling protection for duplicate Content-Length headers and add corresponding tests

httplib.h

@@ -6813,6 +6813,16 @@ inline bool read_headers(Stream &strm, Headers &headers) {
     header_count++;
   }
 
+  // RFC 9110 Section 8.6: Reject requests with multiple Content-Length
+  // headers that have different values to prevent request smuggling.
+  auto cl_range = headers.equal_range("Content-Length");
+  if (cl_range.first != cl_range.second) {
+    const auto &first_val = cl_range.first->second;
+    for (auto it = std::next(cl_range.first); it != cl_range.second; ++it) {
+      if (it->second != first_val) { return false; }
+    }
+  }
+
   return true;
 }