WebSocket and Dynamic Thread Pool support (#2368)

* WebSocket support

* Validate selected subprotocol in WebSocket handshake

* Fix problem with a Unit test

* Dynamic Thread Pool support

* Fix race condition in new Dynamic ThreadPool

test/test_websocket_heartbeat.cc

@@ -0,0 +1,77 @@
+// Standalone test for WebSocket automatic heartbeat.
+// Compiled with a 1-second ping interval so we can verify heartbeat behavior
+// without waiting 30 seconds.
+
+#define CPPHTTPLIB_WEBSOCKET_PING_INTERVAL_SECOND 1
+#define CPPHTTPLIB_WEBSOCKET_READ_TIMEOUT_SECOND 3
+#include <httplib.h>
+
+#include "gtest/gtest.h"
+
+using namespace httplib;
+
+class WebSocketHeartbeatTest : public ::testing::Test {
+protected:
+  void SetUp() override {
+    svr_.WebSocket("/ws", [](const Request &, ws::WebSocket &ws) {
+      std::string msg;
+      while (ws.read(msg)) {
+        ws.send(msg);
+      }
+    });
+
+    port_ = svr_.bind_to_any_port("localhost");
+    thread_ = std::thread([this]() { svr_.listen_after_bind(); });
+    svr_.wait_until_ready();
+  }
+
+  void TearDown() override {
+    svr_.stop();
+    thread_.join();
+  }
+
+  Server svr_;
+  int port_;
+  std::thread thread_;
+};
+
+// Verify that an idle connection stays alive beyond the read timeout
+// thanks to automatic heartbeat pings.
+TEST_F(WebSocketHeartbeatTest, IdleConnectionStaysAlive) {
+  ws::WebSocketClient client("ws://localhost:" + std::to_string(port_) + "/ws");
+  ASSERT_TRUE(client.connect());
+
+  // Sleep longer than read timeout (3s). Without heartbeat, the connection
+  // would time out. With heartbeat pings every 1s, it stays alive.
+  std::this_thread::sleep_for(std::chrono::seconds(5));
+
+  // Connection should still be open
+  ASSERT_TRUE(client.is_open());
+
+  // Verify we can still exchange messages
+  ASSERT_TRUE(client.send("hello after idle"));
+  std::string msg;
+  ASSERT_TRUE(client.read(msg));
+  EXPECT_EQ("hello after idle", msg);
+
+  client.close();
+}
+
+// Verify that multiple heartbeat cycles work
+TEST_F(WebSocketHeartbeatTest, MultipleHeartbeatCycles) {
+  ws::WebSocketClient client("ws://localhost:" + std::to_string(port_) + "/ws");
+  ASSERT_TRUE(client.connect());
+
+  // Wait through several heartbeat cycles
+  for (int i = 0; i < 3; i++) {
+    std::this_thread::sleep_for(std::chrono::milliseconds(1500));
+    ASSERT_TRUE(client.is_open());
+    std::string text = "msg" + std::to_string(i);
+    ASSERT_TRUE(client.send(text));
+    std::string msg;
+    ASSERT_TRUE(client.read(msg));
+    EXPECT_EQ(text, msg);
+  }
+
+  client.close();
+}