fix: increment mSeq on queue drop and fix flaky connection-refused test

- Advance mSeq when dropping events so consumers can detect gaps via
  sequence numbers, and log the dropped seq
- Use ephemeral port (bind + close) instead of hardcoded 19999 for the
  connection-refused test to avoid false negatives on busy machines

src/ripple/net/impl/RPCSub.cpp

@@ -80,8 +80,10 @@ public:
 
         if (mDeque.size() >= maxQueueSize)
         {
-            JLOG(j_.warn()) << "RPCCall::fromNetwork drop: queue full ("
-                            << mDeque.size() << "), endpoint=" << mIp;
+            JLOG(j_.warn())
+                << "RPCCall::fromNetwork drop: queue full (" << mDeque.size()
+                << "), seq=" << mSeq << ", endpoint=" << mIp;
+            ++mSeq;
             return;
         }
 

src/test/net/HTTPClient_test.cpp

@@ -321,14 +321,21 @@ class HTTPClient_test : public beast::unit_test::suite
         using namespace jtx;
         Env env{*this};
 
-        // No server listening — connection refused.
-        // Use a port that's very unlikely to be in use.
+        // Bind a port, then close it — guarantees nothing is listening.
+        boost::asio::io_service tmp;
+        boost::asio::ip::tcp::acceptor acc(
+            tmp,
+            boost::asio::ip::tcp::endpoint(
+                boost::asio::ip::address::from_string("127.0.0.1"), 0));
+        auto port = acc.local_endpoint().port();
+        acc.close();
+
         std::atomic<int> completed{0};
         auto j = env.app().journal("HTTPClient");
 
         {
             boost::asio::io_service ios;
-            fireRequest(ios, "127.0.0.1", 19999, completed, j);
+            fireRequest(ios, "127.0.0.1", port, completed, j);
             ios.run();
         }