fix: resolve Clang and MSVC CI build failures

Clang (tests-clang):
- Switch from clang-15 to clang-18 (available in Ubuntu noble's standard
  repos — no extra PPA needed)
- Remove g++-15 install from Clang job: mixing clang-15 with GCC 15's
  stdlib caused std::ranges::empty_view concept resolution failures;
  clang-18 + default GCC 14 stdlib is a well-tested combination

MSVC (tests-msvc) and all compilers:
- Add add_compile_definitions(BOOST_UT_DISABLE_MODULE) in tests/CMakeLists.txt
  Boost.UT v2.1.0 unconditionally emits 'export module boost.ut' when
  __cpp_modules is defined, causing fatal error C3378 under MSVC
  (/std:c++latest). Disabling module mode forces header-only inclusion
  which works with all three compilers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Pierre-Luc Gagné

.github/workflows/ci.yml

@@ -88,16 +88,15 @@ jobs:
             | sudo tee /usr/share/keyrings/kitware-archive-keyring.gpg > /dev/null
           echo 'deb [signed-by=/usr/share/keyrings/kitware-archive-keyring.gpg] https://apt.kitware.com/ubuntu/ noble main' \
             | sudo tee /etc/apt/sources.list.d/kitware.list
-          sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
           sudo apt-get update -q
-          sudo apt-get install -y cmake ninja-build clang-15 \
+          sudo apt-get install -y cmake ninja-build clang-18 \
             libmysqlclient-dev pkg-config
 
       - name: Configure
         run: cmake --preset release -DSKIP_DOCKER_MANAGEMENT=ON -DBUILD_INTEGRATION_TESTS=OFF -DBUILD_EXAMPLES=OFF
         env:
-          CXX: clang++-15
-          CC: clang-15
+          CXX: clang++-18
+          CC: clang-18
 
       - name: Build
         run: cmake --build build -j4

tests/CMakeLists.txt

@@ -14,6 +14,10 @@ if(NOT TARGET Boost::ut)
     FetchContent_MakeAvailable(boost_ut)
 endif()
 
+# Disable boost.ut's C++20 module mode — not all compilers/toolchains support
+# its module declaration (notably MSVC issues C3378 with /std:c++latest).
+add_compile_definitions(BOOST_UT_DISABLE_MODULE)
+
 # Unit tests
 add_subdirectory(unit)