HttpClient using libcurl, Unix Sockets for HTTP. HTTPS support (#770)

The main goal of this change is to eliminate the cpr back-end altogether and replace it with the curl implementation. I would expect to drop cpr as soon as we feel happy with the libcurl back-end. That would leave us with a direct dependency on libcurl only, and cpr can be eliminated as a dependency.

### HttpClient Backend Overhaul
- Implemented a new **libcurl-based HttpClient** backend (`httpclientcurl.cpp`, ~2000 lines)
  as an alternative to the cpr-based one
- Made HttpClient backend **configurable at runtime** via constructor arguments
  and `-httpclient=...` CLI option (for zen, zenserver, and tests)
- Extended HttpClient test suite to cover multipart/content-range scenarios

### Unix Domain Socket Support
- Added Unix domain socket support to **httpasio** (server side)
- Added Unix domain socket support to **HttpClient**
- Added Unix domain socket support to **HttpWsClient** (WebSocket client)
- Templatized `HttpServerConnectionT<SocketType>` and `WsAsioConnectionT<SocketType>`
  to handle TCP, Unix, and SSL sockets uniformly via `if constexpr` dispatch

### HTTPS Support
- Added **preliminary HTTPS support to httpasio** (for Mac/Linux via OpenSSL)
- Added **basic HTTPS support for http.sys** (Windows)
- Implemented HTTPS test for httpasio
- Split `InitializeServer` into smaller sub-functions for http.sys

### Other Notable Changes
- Improved **zenhttp-test stability** with dynamic port allocation
- Enhanced port retry logic in http.sys (handles ERROR_ACCESS_DENIED)
- Fatal signal/exception handlers for backtrace generation in tests
- Added `zen bench http` subcommand to exercise network + HTTP client/server communication stack

1 files changed, 71 insertions, 2 deletions

diff --git a/src/zenhttp/servers/wstest.cpp b/src/zenhttp/servers/wstest.cpp
index 2134e4ff1..042afd8ff 100644
--- a/src/zenhttp/servers/wstest.cpp
+++ b/src/zenhttp/servers/wstest.cpp
@@ -485,7 +485,7 @@ TEST_CASE("websocket.integration")
 
 	Ref<HttpServer> Server = CreateHttpAsioServer(AsioConfig{});
 
-	int Port = Server->Initialize(7575, TmpDir.Path());
+	int Port = Server->Initialize(0, TmpDir.Path());
 	REQUIRE(Port != 0);
 
 	Server->RegisterService(TestService);
@@ -797,7 +797,7 @@ TEST_CASE("websocket.client")
 
 	Ref<HttpServer> Server = CreateHttpAsioServer(AsioConfig{});
 
-	int Port = Server->Initialize(7576, TmpDir.Path());
+	int Port = Server->Initialize(0, TmpDir.Path());
 	REQUIRE(Port != 0);
 
 	Server->RegisterService(TestService);
@@ -913,6 +913,75 @@ TEST_CASE("websocket.client")
 	}
 }
 
+TEST_CASE("websocket.client.unixsocket")
+{
+	WsTestService			 TestService;
+	ScopedTemporaryDirectory TmpDir;
+	std::string				 SocketPath = (TmpDir.Path() / "ws.sock").string();
+
+	Ref<HttpServer> Server = CreateHttpAsioServer(AsioConfig{.UnixSocketPath = SocketPath});
+
+	int Port = Server->Initialize(0, TmpDir.Path());
+	REQUIRE(Port != 0);
+
+	Server->RegisterService(TestService);
+
+	std::thread ServerThread([&]() { Server->Run(false); });
+
+	auto ServerGuard = MakeGuard([&]() {
+		Server->RequestExit();
+		if (ServerThread.joinable())
+		{
+			ServerThread.join();
+		}
+		Server->Close();
+	});
+
+	Sleep(100);
+
+	SUBCASE("connect, echo, close over unix socket")
+	{
+		TestWsClientHandler	 Handler;
+		HttpWsClientSettings Settings;
+		Settings.UnixSocketPath = SocketPath;
+
+		HttpWsClient Client("ws://localhost/wstest/ws", Handler, Settings);
+		Client.Connect();
+
+		// Wait for OnWsOpen
+		auto Deadline = std::chrono::steady_clock::now() + 5s;
+		while (Handler.m_OpenCount.load() == 0 && std::chrono::steady_clock::now() < Deadline)
+		{
+			Sleep(10);
+		}
+		REQUIRE_EQ(Handler.m_OpenCount.load(), 1);
+		CHECK(Client.IsOpen());
+
+		// Send text, expect echo
+		Client.SendText("hello over unix socket");
+
+		Deadline = std::chrono::steady_clock::now() + 5s;
+		while (Handler.m_MessageCount.load() == 0 && std::chrono::steady_clock::now() < Deadline)
+		{
+			Sleep(10);
+		}
+		CHECK_EQ(Handler.m_MessageCount.load(), 1);
+		CHECK_EQ(Handler.m_LastMessage, "hello over unix socket");
+
+		// Close
+		Client.Close(1000, "done");
+
+		Deadline = std::chrono::steady_clock::now() + 5s;
+		while (Handler.m_CloseCount.load() == 0 && std::chrono::steady_clock::now() < Deadline)
+		{
+			Sleep(10);
+		}
+
+		Sleep(50);
+		CHECK_FALSE(Client.IsOpen());
+	}
+}
+
 TEST_SUITE_END();
 
 void