Add early long path validation to Windows build script

eng/build.ps1

@@ -135,6 +135,29 @@ function Get-Help() {
   Write-Host "For more information, check out https://github.com/dotnet/runtime/blob/main/docs/workflow/README.md"
 }
 
+function Check-LongPathSupport() {
+  $regValue = Get-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Control\FileSystem" -Name "LongPathsEnabled" -ErrorAction SilentlyContinue
+
+  if ($regValue -and $regValue.LongPathsEnabled -eq 1) {
+    return
+  }
+
+  Write-Host ""
+  Write-Host "ERROR: Long file paths are not enabled on this system." -ForegroundColor Red
+  Write-Host "The dotnet/runtime repository requires long path support to build successfully." -ForegroundColor Red
+  Write-Host ""
+  Write-Host "Please follow the instructions at:" -ForegroundColor Yellow
+  Write-Host "  docs\workflow\requirements\windows-requirements.md#enable-long-paths" -ForegroundColor Yellow
+  Write-Host ""
+  Write-Host "Quick fix:" -ForegroundColor Yellow
+  Write-Host "  1. Enable long paths in Windows (requires admin):" -ForegroundColor White
+  Write-Host "     https://learn.microsoft.com/windows/win32/fileio/maximum-file-path-limitation" -ForegroundColor White
+  Write-Host "  2. Enable long paths in Git:" -ForegroundColor White
+  Write-Host "     git config --system core.longpaths true" -ForegroundColor White
+  Write-Host ""
+  exit 1
+}
+
 if ($help) {
   Get-Help
   exit 0
@@ -152,6 +175,8 @@ if ($subset -eq 'help') {
   exit 0
 }
 
+Check-LongPathSupport
+
 # Lower-case the passed in OS string.
 if ($os) {
   $os = $os.ToLowerInvariant()