fix: improve wheel download script reliability

scripts/download_wheels.ps1

@@ -1,5 +1,6 @@
 param(
-  [switch]$NoPause
+  [switch]$NoPause,
+  [string]$IndexUrl = "https://pypi.org/simple"
 )
 
 $ErrorActionPreference = "Stop"
@@ -23,6 +24,27 @@ function Get-PythonCommand {
   throw "Python was not found. Install Python 3.10+ before downloading wheels."
 }
 
+function Invoke-PipDownload {
+  param(
+    [string]$PythonCommand,
+    [string[]]$PythonArgs
+  )
+
+  $pipArgs = @(
+    "-m", "pip", "download",
+    "--index-url", $IndexUrl,
+    "--trusted-host", "pypi.org",
+    "--trusted-host", "files.pythonhosted.org",
+    "--disable-pip-version-check",
+    "--retries", "3",
+    "--timeout", "60",
+    "-r", $Req,
+    "-d", $WheelDir
+  )
+
+  & $PythonCommand @PythonArgs @pipArgs
+}
+
 try {
   Set-Location $Root
   if (-not (Test-Path $Req)) {
@@ -34,13 +56,13 @@ try {
 
   Write-Host "Downloading offline wheels to: $WheelDir" -ForegroundColor Cyan
   if ($python -eq "py") {
-    & py -3 -m pip download -r $Req -d $WheelDir
+    Invoke-PipDownload -PythonCommand "py" -PythonArgs @("-3")
   } else {
-    & $python -m pip download -r $Req -d $WheelDir
+    Invoke-PipDownload -PythonCommand $python -PythonArgs @()
   }
 
   if ($LASTEXITCODE -ne 0) {
-    throw "Wheel download failed. Check network access and pip."
+    throw "Wheel download failed. Check network access to $IndexUrl. If your environment uses a private PyPI mirror, pass -IndexUrl with the mirror address."
   }
 
   Write-Host ""