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, 4 deletions

diff --git a/src/zen/zen.cpp b/src/zen/zen.cpp
index 9a466da2e..86c29344e 100644
--- a/src/zen/zen.cpp
+++ b/src/zen/zen.cpp
@@ -196,6 +196,7 @@ ZenCmdBase::GetSubCommand(cxxopts::Options&,
 
 ZenSubCmdBase::ZenSubCmdBase(std::string_view Name, std::string_view Description)
 : m_SubOptions(std::string(Name), std::string(Description))
+, m_Description(Description)
 {
 	m_SubOptions.add_options()("h,help", "Print help");
 }
@@ -213,6 +214,35 @@ ZenCmdWithSubCommands::OnParentOptionsParsed(const ZenCliOptions& /*GlobalOption
 }
 
 void
+ZenCmdWithSubCommands::PrintHelp()
+{
+	// Show all option groups except the internal "__hidden__" group used to
+	// silently capture positional arguments.
+	std::vector<std::string> Groups = Options().groups();
+	Groups.erase(std::remove(Groups.begin(), Groups.end(), std::string("__hidden__")), Groups.end());
+
+	Options().set_width(TuiConsoleColumns(80));
+	printf("%s\n", Options().help(Groups).c_str());
+
+	// Append subcommand listing.
+	size_t MaxNameLen = 0;
+	for (ZenSubCmdBase* SubCmd : m_SubCommands)
+	{
+		MaxNameLen = std::max(MaxNameLen, SubCmd->SubOptions().program().size());
+	}
+
+	printf("subcommands:\n");
+	for (ZenSubCmdBase* SubCmd : m_SubCommands)
+	{
+		printf("  %-*s  %s\n",
+			   static_cast<int>(MaxNameLen),
+			   SubCmd->SubOptions().program().c_str(),
+			   std::string(SubCmd->Description()).c_str());
+	}
+	printf("\nFor global options run: zen --help\n");
+}
+
+void
 ZenCmdWithSubCommands::Run(const ZenCliOptions& GlobalOptions, int argc, char** argv)
 {
 	std::vector<cxxopts::Options*> SubOptionPtrs;
@@ -226,15 +256,47 @@ ZenCmdWithSubCommands::Run(const ZenCliOptions& GlobalOptions, int argc, char**
 	std::vector<char*> SubCommandArguments;
 	int				   ParentArgc = GetSubCommand(Options(), argc, argv, SubOptionPtrs, MatchedSubOption, SubCommandArguments);
 
-	if (!ParseOptions(Options(), ParentArgc, argv))
+	// Intercept --help/-h in the parent arg range before calling ParseOptions so
+	// we can append subcommand information to the output.  When a subcommand was
+	// found argv[ParentArgc-1] is the subcommand name itself, which we exclude.
+	int ParentArgEnd = (MatchedSubOption != nullptr) ? ParentArgc - 1 : ParentArgc;
+	for (int i = 1; i < ParentArgEnd; ++i)
 	{
-		return;
+		std::string_view Arg(argv[i]);
+		if (Arg == "--help" || Arg == "-h")
+		{
+			PrintHelp();
+			return;
+		}
+	}
+
+	// Parse parent options.  When a subcommand was matched we strip its name from
+	// the arg list so the parent parser does not see it as an unmatched positional.
+	if (MatchedSubOption != nullptr)
+	{
+		std::vector<char*> ParentArgs;
+		ParentArgs.reserve(static_cast<size_t>(ParentArgc - 1));
+		ParentArgs.push_back(argv[0]);
+		std::copy(argv + 1, argv + ParentArgc - 1, std::back_inserter(ParentArgs));
+		if (!ParseOptions(Options(), static_cast<int>(ParentArgs.size()), ParentArgs.data()))
+		{
+			return;
+		}
+	}
+	else
+	{
+		if (!ParseOptions(Options(), ParentArgc, argv))
+		{
+			return;
+		}
 	}
 
 	if (MatchedSubOption == nullptr)
 	{
+		PrintHelp();
+
 		ExtendableStringBuilder<128> VerbList;
-		for (bool First = true; ZenSubCmdBase * SubCmd : m_SubCommands)
+		for (bool First = true; ZenSubCmdBase* SubCmd : m_SubCommands)
 		{
 			if (!First)
 			{
@@ -243,7 +305,7 @@ ZenCmdWithSubCommands::Run(const ZenCliOptions& GlobalOptions, int argc, char**
 			VerbList.Append(SubCmd->SubOptions().program());
 			First = false;
 		}
-		throw OptionParseException(fmt::format("No subcommand specified. Available subcommands: {}", VerbList.ToView()), Options().help());
+		throw OptionParseException(fmt::format("No subcommand specified. Available subcommands: {}", VerbList.ToView()), {});
 	}
 
 	ZenSubCmdBase* MatchedSubCmd = nullptr;
@@ -621,6 +683,9 @@ main(int argc, char** argv)
 	Options.add_options()("malloc", "Configure memory allocator subsystem", cxxopts::value(MemoryOptions)->default_value("mimalloc"));
 	Options.add_options()("help", "Show command line help");
 	Options.add_options()("c, command", "Sub command", cxxopts::value<std::string>(SubCommand));
+	Options.add_options()("httpclient",
+						  "Select HTTP client implementation (e.g. 'curl', 'cpr')",
+						  cxxopts::value<std::string>(GlobalOptions.HttpClientBackend)->default_value("cpr"));
 
 	int CoreLimit = 0;
 
@@ -783,6 +848,8 @@ main(int argc, char** argv)
 			FreeCallstack(Callstack);
 		});
 
+		zen::SetDefaultHttpClientBackend(GlobalOptions.HttpClientBackend);
+
 		zen::MaximizeOpenFileCount();
 
 		//////////////////////////////////////////////////////////////////////////