implement new Download-FileDirectly function to handle downloads

tests/Get-DSAManifest.Tests.ps1

@@ -9,7 +9,8 @@ $null = [Management.Automation.Language.Parser]::ParseFile($DsaScriptUnderTest,
 If ($DsaErrors.Count) { Throw ($DsaErrors | Out-String) }
 
 Function Assert-Dsa { param([bool]$Condition, [string]$Description) If (!$Condition) { Throw $Description } }
-Function Download-File {
+Function Download-File { Throw 'DSA must use Download-FileDirectly.' }
+Function Download-FileDirectly {
     [CmdletBinding()]
     param($URL, $File)
     $script:DsaDownloadCalls++
@@ -80,6 +81,7 @@ Function Invoke-DsaCase {
         Assert-Dsa ($script:DsaDownloadCalls -eq 0) 'Invalid flag must fail before downloading.'
         Return
     }
+    Assert-Dsa ($script:DsaDownloadCalls -eq 1) 'Expected one direct download.'
     If ($Mode -in @('empty-download','download-failure')) {
         Assert-Dsa ($null -ne $DsaCaught) 'Download failure must stop collection.'
         Assert-Dsa ($script:DsaProcessCalls -eq 0 -and $script:DsaRecordedUploads.Count -eq 0) 'Download failure must prevent execution and upload.'