ci(windows): make VS2026 configure idempotent for build-cache hits

The VS2026 configure step used "mkdir -p" under PowerShell, where mkdir maps to
New-Item, which errors when the directory already exists. On a build-cache hit
the restored build/<config> directory is present, so configure failed
intermittently (cache miss passed, cache hit failed). Use
New-Item -ItemType Directory -Force, matching the VS2022 job.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: wysaid

.github/workflows/windows-build.yml

@@ -314,8 +314,10 @@ jobs:
         
         # Check CMake version
         cmake --version
-        
-        mkdir -p "build/${{ matrix.config }}-${{ matrix.library_type }}"
+
+        # Use New-Item -Force (idempotent): in PowerShell "mkdir -p" maps to New-Item,
+        # which errors when the directory already exists -- e.g. after a build-cache hit.
+        New-Item -ItemType Directory -Force -Path "build/${{ matrix.config }}-${{ matrix.library_type }}" | Out-Null
         cd "build/${{ matrix.config }}-${{ matrix.library_type }}"
         
         # Use the Visual Studio 18 2026 generator with its default toolset.