fix(build): require VC C++ tools in vswhere query

Two 18.x installs coexisted: VS2026 Community (full IDE, has VC v180)
and a BuildTools at (x86)\Microsoft Visual Studio\18\BuildTools that
has MSBuild but NO VC v180 targets. The vswhere query only required
Microsoft.Component.MSBuild, so -latest could (and did) return the
C++-less BuildTools -> MSB4019. Also require
Microsoft.VisualStudio.Component.VC.Tools.x86.x64 so only installs
with the C++ toolset match; the broken BuildTools is excluded and
2026\Community is selected. CI VM BuildTools has the C++ workload so
it still matches.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/legacy/build_plugin_vs2026.py

@@ -152,6 +152,7 @@ def _find_msbuild_via_vswhere(vs_version: str) -> 'Optional[Tuple[Path, str, str
                     '-latest',
                     '-prerelease',
                     '-requires', 'Microsoft.Component.MSBuild',
+                    'Microsoft.VisualStudio.Component.VC.Tools.x86.x64',
                     '-find', r'MSBuild\**\Bin\MSBuild.exe',
                 ],
                 capture_output=True, text=True, timeout=15,