Fix installer checksum verification under Windows PowerShell (#51257)

## Why

A native launcher can pass PowerShell 7 module paths to Windows PowerShell, preventing `Get-FileHash` from loading and blocking archive checksum verification.

## What changed

Update `scripts/install/install.ps1` to compute SHA-256 directly with .NET APIs after resolving the archive path. Dispose the file stream and hash provider, and retain the existing checksum mismatch error.

GitOrigin-RevId: 86e6232bd83c888811fc3b76fcad0aaf046f4bf8

scripts/install/install.ps1

@@ -214,7 +214,21 @@ function Test-ArchiveDigest {
         [string]$ExpectedDigest
     )
 
-    $actualDigest = (Get-FileHash -LiteralPath $ArchivePath -Algorithm SHA256).Hash.ToLowerInvariant()
+    # A native launcher can pass PowerShell 7 module paths to Windows PowerShell,
+    # preventing Get-FileHash from loading. Hash directly without module lookup.
+    $providerPath = $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($ArchivePath)
+    $sha256 = [System.Security.Cryptography.SHA256CryptoServiceProvider]::new()
+    try {
+        $stream = [System.IO.File]::OpenRead($providerPath)
+        try {
+            $actualDigest = [System.BitConverter]::ToString($sha256.ComputeHash($stream)).Replace("-", "").ToLowerInvariant()
+        } finally {
+            $stream.Dispose()
+        }
+    } finally {
+        $sha256.Dispose()
+    }
+
     if ($actualDigest -ne $ExpectedDigest) {
         throw "Downloaded Codex archive checksum did not match expected digest. Expected $ExpectedDigest but got $actualDigest."
     }