[garethsb]

The standard only defines std::char_traits for the character types. libc++ 19 removed the generic std::char_traits template, so std::basic_string<uint8_t> no longer compiles. cpprest/streams.h used it in Value2StringFormatter<uint8_t>. As a result, every file that includes that header failed to compile with libc++ 19 or later. That includes the default Xcode on macOS since Xcode 16.3, and Clang with -stdlib=libc++ elsewhere.

This change avoids std::basic_string<uint8_t> rather than defining character traits for uint8_t:

• Value2StringFormatter<uint8_t> returns the formatted bytes as a std::string, and basic_ostream writes any formatted string through a private helper.
• The basic_ostream::print overload for strings is now a template on the traits and allocator. Calling print on a basic_ostream<uint8_t> therefore no longer instantiates std::basic_string<uint8_t> during overload resolution. For char and wchar_t streams, partial ordering still selects this overload for strings, as before.
• Printing to a basic_ostream<uint8_t> no longer stops at an embedded NUL byte, because the bytes are no longer copied through c_str().
• In the stream buffer tests, std::vector replaces std::basic_string for the data to write. One test used rawptr_buffer<std::string> where it meant rawptr_buffer<char>.

The second commit adds macOS jobs to CI, on the macos-15 (Xcode 16.4) and macos-26 (Xcode 26.6) runners, with Homebrew's Boost and OpenSSL. They exclude websockets, because Homebrew's websocketpp is 0.8.2, which does not build with Boost 1.87 or later.

Tested locally:

• Ubuntu 26.04, Clang 21 with libc++ 21: before the change, 94 files fail to compile. After it, the whole tree builds. The tests cannot run in that container, because Ubuntu's Boost is built against libstdc++ and the test runner uses boost::filesystem to find the test libraries. The macOS jobs run the tests.
• Ubuntu 24.04, GCC 13 with AddressSanitizer: all 237 streams tests pass, as do the json, utils, uri, pplx and listener tests.
• Windows, MSVC 14.50, Debug: all 270 streams tests pass.

Fixes #11.

The same problem is reported upstream in microsoft#1812. microsoft#1829 fixes it with a separate traits class for uint8_t, and Homebrew and FreeBSD apply that change as patches. This change is smaller and does not need a traits class.

Co-authored-by: Gareth's 🤖 buddy