Preserve strict credential gate for real regression mode

Real regression runs that explicitly pass -RequireCredentials should still fail fast when credentials cannot be confirmed. This keeps the real-ASR signal strict while leaving the default runtime smoke suite decoupled from legacy credentials.json.\n\nConstraint: default smoke suite must not depend on migrated plaintext JSON.\nRejected: Keep warning-only behavior in real regression mode | allows credential-missing runs to appear as valid real-ASR regressions.\nConfidence: high\nScope-risk: narrow\nDirective: Keep -RequireCredentials in windows-real-regression as a hard gate unless credential-vault probing is added.\nTested: git diff --check\nNot-tested: Running Windows PowerShell regression scripts in Linux environment.\nRelated: #230

openless-all/app/scripts/windows-real-regression.ps1

@@ -84,7 +84,7 @@ $credentialStatus = Get-OpenLessCredentialStatus
 Write-Host "== Credential gate =="
 $credentialStatus | Format-List
 if ($RequireCredentials -and (-not $credentialStatus.VolcengineConfigured -or -not $credentialStatus.ArkConfigured)) {
-  Write-Warning "Legacy credentials.json is incomplete; continuing because the app uses the OS credential vault."
+  throw "Real regression requires configured Volcengine ASR and Ark LLM credentials."
 }
 
 Write-Host ""