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, 117 insertions, 4 deletions

diff --git a/src/zenhttp/httpclient.cpp b/src/zenhttp/httpclient.cpp
index 281d512cf..9baf4346e 100644
--- a/src/zenhttp/httpclient.cpp
+++ b/src/zenhttp/httpclient.cpp
@@ -40,6 +40,35 @@ extern HttpClientBase* CreateCprHttpClient(std::string_view			 BaseUri,
 										   const HttpClientSettings& ConnectionSettings,
 										   std::function<bool()>&&	 CheckIfAbortFunction);
 
+extern HttpClientBase* CreateCurlHttpClient(std::string_view		  BaseUri,
+											const HttpClientSettings& ConnectionSettings,
+											std::function<bool()>&&	  CheckIfAbortFunction);
+
+static HttpClientBackend g_DefaultHttpClientBackend = HttpClientBackend::kCpr;
+
+void
+SetDefaultHttpClientBackend(HttpClientBackend Backend)
+{
+	g_DefaultHttpClientBackend = Backend;
+}
+
+void
+SetDefaultHttpClientBackend(std::string_view Backend)
+{
+	if (Backend == "cpr")
+	{
+		g_DefaultHttpClientBackend = HttpClientBackend::kCpr;
+	}
+	else if (Backend == "curl")
+	{
+		g_DefaultHttpClientBackend = HttpClientBackend::kCurl;
+	}
+	else
+	{
+		g_DefaultHttpClientBackend = HttpClientBackend::kDefault;
+	}
+}
+
 using namespace std::literals;
 
 //////////////////////////////////////////////////////////////////////////
@@ -104,6 +133,71 @@ HttpClientBase::GetAccessToken()
 
 //////////////////////////////////////////////////////////////////////////
 
+HttpClientError::ResponseClass
+HttpClientError::GetResponseClass() const
+{
+	if (m_Error != HttpClientErrorCode::kOK)
+	{
+		switch (m_Error)
+		{
+			case HttpClientErrorCode::kConnectionFailure:
+				return ResponseClass::kHttpCantConnectError;
+			case HttpClientErrorCode::kHostResolutionFailure:
+			case HttpClientErrorCode::kProxyResolutionFailure:
+				return ResponseClass::kHttpNoHost;
+			case HttpClientErrorCode::kInternalError:
+			case HttpClientErrorCode::kNetworkReceiveError:
+			case HttpClientErrorCode::kNetworkSendFailure:
+			case HttpClientErrorCode::kOperationTimedOut:
+				return ResponseClass::kHttpTimeout;
+			case HttpClientErrorCode::kSSLConnectError:
+			case HttpClientErrorCode::kSSLCertificateError:
+			case HttpClientErrorCode::kSSLCACertError:
+			case HttpClientErrorCode::kGenericSSLError:
+				return ResponseClass::kHttpSLLError;
+			default:
+				return ResponseClass::kHttpOtherClientError;
+		}
+	}
+	else if (IsHttpSuccessCode(m_ResponseCode))
+	{
+		return ResponseClass::kSuccess;
+	}
+	else
+	{
+		switch (m_ResponseCode)
+		{
+			case HttpResponseCode::Unauthorized:
+				return ResponseClass::kHttpUnauthorized;
+			case HttpResponseCode::NotFound:
+				return ResponseClass::kHttpNotFound;
+			case HttpResponseCode::Forbidden:
+				return ResponseClass::kHttpForbidden;
+			case HttpResponseCode::Conflict:
+				return ResponseClass::kHttpConflict;
+			case HttpResponseCode::InternalServerError:
+				return ResponseClass::kHttpInternalServerError;
+			case HttpResponseCode::ServiceUnavailable:
+				return ResponseClass::kHttpServiceUnavailable;
+			case HttpResponseCode::BadGateway:
+				return ResponseClass::kHttpBadGateway;
+			case HttpResponseCode::GatewayTimeout:
+				return ResponseClass::kHttpGatewayTimeout;
+			default:
+				if (m_ResponseCode >= HttpResponseCode::InternalServerError)
+				{
+					return ResponseClass::kHttpOtherServerError;
+				}
+				else
+				{
+					return ResponseClass::kHttpOtherClientError;
+				}
+		}
+	}
+}
+
+//////////////////////////////////////////////////////////////////////////
+
 std::vector<std::pair<uint64_t, uint64_t>>
 HttpClient::Response::GetRanges(std::span<const std::pair<uint64_t, uint64_t>> OffsetAndLengthPairs) const
 {
@@ -222,7 +316,11 @@ HttpClient::Response::ErrorMessage(std::string_view Prefix) const
 {
 	if (Error.has_value())
 	{
-		return fmt::format("{}{}HTTP error ({}) '{}'", Prefix, Prefix.empty() ? ""sv : ": "sv, Error->ErrorCode, Error->ErrorMessage);
+		return fmt::format("{}{}HTTP error ({}) '{}'",
+						   Prefix,
+						   Prefix.empty() ? ""sv : ": "sv,
+						   static_cast<int>(Error->ErrorCode),
+						   Error->ErrorMessage);
 	}
 	else if (StatusCode != HttpResponseCode::ImATeapot && (int)StatusCode)
 	{
@@ -245,19 +343,34 @@ HttpClient::Response::ThrowError(std::string_view ErrorPrefix)
 {
 	if (!IsSuccess())
 	{
-		throw HttpClientError(ErrorMessage(ErrorPrefix), Error.has_value() ? Error.value().ErrorCode : 0, StatusCode);
+		throw HttpClientError(ErrorMessage(ErrorPrefix),
+							  Error.has_value() ? Error.value().ErrorCode : HttpClientErrorCode::kOK,
+							  StatusCode);
 	}
 }
 
 //////////////////////////////////////////////////////////////////////////
 
 HttpClient::HttpClient(std::string_view BaseUri, const HttpClientSettings& ConnectionSettings, std::function<bool()>&& CheckIfAbortFunction)
-: m_BaseUri(BaseUri)
+: m_Log(zen::logging::Get(ConnectionSettings.LogCategory))
+, m_BaseUri(BaseUri)
 , m_ConnectionSettings(ConnectionSettings)
 {
 	m_SessionId = GetSessionIdString();
 
-	m_Inner = CreateCprHttpClient(BaseUri, ConnectionSettings, std::move(CheckIfAbortFunction));
+	HttpClientBackend EffectiveBackend =
+		ConnectionSettings.Backend != HttpClientBackend::kDefault ? ConnectionSettings.Backend : g_DefaultHttpClientBackend;
+
+	switch (EffectiveBackend)
+	{
+		case HttpClientBackend::kCurl:
+			m_Inner = CreateCurlHttpClient(BaseUri, ConnectionSettings, std::move(CheckIfAbortFunction));
+			break;
+		case HttpClientBackend::kCpr:
+		default:
+			m_Inner = CreateCprHttpClient(BaseUri, ConnectionSettings, std::move(CheckIfAbortFunction));
+			break;
+	}
 }
 
 HttpClient::~HttpClient()