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, 41 insertions, 0 deletions

diff --git a/src/zenserver-test/zenserver-test.cpp b/src/zenserver-test/zenserver-test.cpp
index 8d5400294..0b2bc50c0 100644
--- a/src/zenserver-test/zenserver-test.cpp
+++ b/src/zenserver-test/zenserver-test.cpp
@@ -24,6 +24,10 @@
 #	include <atomic>
 #	include <filesystem>
 
+ZEN_THIRD_PARTY_INCLUDES_START
+#	include <asio.hpp>
+ZEN_THIRD_PARTY_INCLUDES_END
+
 #	if ZEN_PLATFORM_WINDOWS
 #		include <ppl.h>
 #		include <process.h>
@@ -109,6 +113,11 @@ main(int argc, char** argv)
 				ServerClass = argv[++i];
 			}
 		}
+		else if (std::string_view Arg(argv[i]); Arg.starts_with("--httpclient="sv))
+		{
+			std::string_view Value = Arg.substr(13);
+			zen::SetDefaultHttpClientBackend(Value);
+		}
 		else if (argv[i] == "--verbose"sv)
 		{
 			Verbose = true;
@@ -341,6 +350,38 @@ TEST_CASE("http.package")
 	CHECK_EQ(ResponsePackage, TestPackage);
 }
 
+#	if defined(ASIO_HAS_LOCAL_SOCKETS)
+TEST_CASE("http.unixsocket")
+{
+	std::filesystem::path TestDir	 = TestEnv.CreateNewTestDir();
+	std::filesystem::path SocketDir	 = TestEnv.CreateNewTestDir();
+	std::string			  SocketPath = (SocketDir / "zen.sock").string();
+
+	ZenServerInstance Instance(TestEnv);
+	Instance.SetDataDir(TestDir);
+	const uint16_t PortNumber = Instance.SpawnServerAndWaitUntilReady(fmt::format("--http=asio --unix-socket {}", SocketPath));
+
+	// Connect via Unix socket (BaseUri still needed for Host header)
+	HttpClientSettings Settings;
+	Settings.UnixSocketPath = SocketPath;
+	HttpClient Http{fmt::format("http://localhost:{}", PortNumber), Settings, {}};
+
+	SUBCASE("GET over unix socket")
+	{
+		HttpClient::Response Res = Http.Get("/testing/hello");
+		CHECK(Res.IsSuccess());
+	}
+
+	SUBCASE("POST echo over unix socket")
+	{
+		IoBuffer			 Body{IoBuffer::Wrap, "unix-test", 9};
+		HttpClient::Response Res = Http.Post("/testing/echo", Body);
+		CHECK(Res.IsSuccess());
+		CHECK(Res.ResponsePayload.GetView().EqualBytes(Body.GetView()));
+	}
+}
+#	endif
+
 TEST_SUITE_END();
 
 #	if 0