ci(workflows): matrix Windows unit tests on MSVC and clang-cl

Replace the single-compiler `tests-windows-msvc` job with a matrix
job `tests-windows` that runs the Windows shim unit tests under
both native MSVC (cl.exe) and the LLVM clang-cl.exe driver against
the same MSVC headers/CRT. The matrix axis is `compiler: [msvc,
clang-cl]` with `fail-fast: false` so each toolchain reports
independently.

Wire the toolchain selection through the CC/CXX env vars on the
configure step: empty for the MSVC default discovered via vcvars,
or `clang-cl.exe` for the LLVM driver. The remaining steps (build,
ctest, save-ccache) are toolchain-agnostic.

Catches toolchain-specific shim regressions (clang-cl widening of
the MSVC shim gates, fuzztest fallback to FetchContent gtest, the
googletest -Wcharacter-conversion suppression) before they hit the
SITL build path.

Signed-off-by: Nuno Marques <n.marques21@hotmail.com>

.github/workflows/checks.yml

@@ -98,14 +98,21 @@ jobs:
         run: git diff --exit-code
         working-directory: src/modules/ekf2/test/change_indication
 
-  tests-windows-msvc:
-    name: Unit Tests (windows-msvc)
+  tests-windows:
+    name: Unit Tests (windows-${{ matrix.compiler }})
     # Native Windows host is required: the gtest binaries under
     # platforms/posix/src/px4/windows/tests/ exercise the _MSC_VER paths
     # in the shim headers and the MSVC-only sources (socket_msvc.cpp,
     # sys/time.cpp, etc.). The shim CMakeLists is gated on WIN32 in
     # platforms/posix/CMakeLists.txt, so a Linux runner cannot stand in.
+    # The compiler axis covers both native MSVC (cl.exe) and the LLVM
+    # clang-cl.exe driver against the same MSVC headers/CRT so we catch
+    # toolchain-specific shim regressions before they hit the SITL build.
     runs-on: windows-2022
+    strategy:
+      fail-fast: false
+      matrix:
+        compiler: [msvc, clang-cl]
     steps:
       - uses: actions/checkout@v6
         with:
@@ -142,11 +149,16 @@ jobs:
       # via px4_posix_windows_add_tests() (see platforms/posix/CMakeLists.txt).
       # Pin Python3_ROOT_DIR to the setup-python interpreter so cmake's
       # find_package(Python3) does not pick the runner image's other Python
-      # where kconfiglib is not installed. We invoke cmake directly here
-      # rather than `make px4_sitl_test` so we only configure and can then
-      # build a narrow set of targets in the next step.
+      # where kconfiglib is not installed. The CC/CXX env vars decide which
+      # toolchain CMake picks: empty for native MSVC (the default cl.exe
+      # discovered via vcvars), or clang-cl.exe for the LLVM driver. We
+      # invoke cmake directly here rather than `make px4_sitl_test` so we
+      # only configure and can then build a narrow set of targets next.
       - name: Configure (px4_sitl_test)
         shell: pwsh
+        env:
+          CC: ${{ matrix.compiler == 'clang-cl' && 'clang-cl' || '' }}
+          CXX: ${{ matrix.compiler == 'clang-cl' && 'clang-cl' || '' }}
         run: |
           cmake -S . -B build/px4_sitl_test -G Ninja `
             -DCONFIG=px4_sitl_test `