[garethsb]

#36 fixed cpprest/json.h for C++23. Clang 21 still gave about 680 warnings about implicit this capture at C++20 and later, many of them from public headers. Three test files also failed to compile at C++20 and later. This PR fixes both and adds a CI job that builds and tests with Clang at C++23.

• Capture this explicitly from C++20. This is Vadim Zeitlin's commit from Fix errors about implicit "this" capture in C++20 mode microsoft/cpprestsdk#1827, cherry-picked with him as the author. It adds CPPREST_CAPTURE_THIS, which expands to , this from C++20, for use as [= CPPREST_CAPTURE_THIS]. Before C++20, [=, this] is ill-formed, and from C++20, [=] capturing this implicitly is deprecated. Two merge conflicts in http_server_asio.cpp came from the Boost 1.87 port's buffer_cast changes, and I resolved them by keeping both changes.
• Define the macro where pplxtasks.h can see it. The cherry-picked commit defines the macros in basic_types.h, which pplxtasks.h doesn't include, so the pplx and streams tests failed to compile. This commit moves them to cpprest_compat.h, which both headers include. The lambda in pplxtasks.h uses only this, so it now uses [this], which is valid from C++11.
• Build the tests at C++20 and later. test_runner.cpp used requires as a variable name, and requires is a keyword from C++20. C++20 also deletes operator<< for writing char16_t to a narrow stream. UnitTest++ checked whether a failure message can show the values with a pre-C++11 overload trick, and that check failed to compile when it selected the deleted operator. It now uses decltype, so a deleted operator counts as unsupported.
• Add a CI job. It runs on Ubuntu 26.04 and builds and tests with Clang at C++23, in Debug and Release.

Testing

I tested with Clang 21.1.8 and libstdc++ 15 on Ubuntu 26.04. Debug builds at C++20 and C++23 pass all 7 test suites, with WebSockets excluded. A C++23 Release build with WebSockets, using Ubuntu's websocketpp package, also passes all 7. Four warnings remain, all in test files: three from implicit conversions in json_numbers_tests.cpp, and one from a deprecated volatile increment in pplxtask_tests.cpp.

Fixes #13.

Co-authored-by: Gareth's 🤖 buddy