Fix Windows task and certificate collector reliability

powershell/Get-ServerCertificates.ps1

@@ -4,12 +4,23 @@ function Get-SamsaraCertificates {
     Invoke-SamsaraRead $Result 'Local machine certificate store list' {
         $collection.stores = @(Get-ChildItem -LiteralPath 'Cert:\LocalMachine' -ErrorAction Stop)
     }
+    # Listing store names is not evidence that any listed store was readable.
+    if ($collection.stores.Count -gt 0) { $Result.successful_subchecks-- }
     foreach ($store in $collection.stores) {
-        Invoke-SamsaraRead $Result 'Local machine public certificate metadata' {
-            Get-ChildItem -LiteralPath $store.PSPath -ErrorAction Stop | ForEach-Object {
-                $Result.data += [pscustomobject]@{ subject = [string]$_.Subject; issuer = [string]$_.Issuer
-                    expires = $_.NotAfter.ToUniversalTime().ToString('o'); thumbprint = [string]$_.Thumbprint
-                    store = [string]$store.PSChildName }
+        # Certificate provider PSPath may omit LocalMachine (including on 5.1).
+        # Construct an explicit provider path from the enumerated store name.
+        $storeName = [string]$store.PSChildName
+        Invoke-SamsaraRead $Result 'Certificate store query' {
+            if ($storeName -notmatch '^[A-Za-z0-9_. -]+$') { throw 'Invalid store name' }
+            $certificates = @(Get-ChildItem -LiteralPath ('Cert:\LocalMachine\' + $storeName) -ErrorAction Stop)
+            foreach ($certificate in $certificates) {
+                Invoke-SamsaraRead $Result 'Certificate metadata projection' {
+                $row = [pscustomobject]@{ subject = [string]$certificate.Subject; issuer = [string]$certificate.Issuer
+                    expires = $certificate.NotAfter.ToUniversalTime().ToString('o'); thumbprint = [string]$certificate.Thumbprint
+                    store = $storeName }
+                $null = ConvertTo-Json -InputObject $row -Depth 4 -ErrorAction Stop
+                $Result.data += $row
+                }
             }
         }
     }