fix(test): harden check-doc-coverage strict-mode assertion

Match Exit-WithCode output without assuming Invoke-TestScriptFile exposes
an Error property on the result object.

Author: bolens

tests/unit/validation/check/validation-check-doc-coverage.tests.ps1

@@ -71,7 +71,7 @@ function Get-DocCoverageStrictFixture {
             )
 
             $result.ExitCode | Should -Be 1
-            $combined = @($result.Output; $result.Error; $result.StdErr) -join "`n"
-            $combined | Should -Match 'Missing markdown|blocking issue|Documentation coverage check failed|Exit-WithCode'
+            # Invoke-TestScriptFile merges streams into Output; Exit-WithCode text may vary by host capture.
+            [string]$result.Output | Should -Match 'Missing markdown|blocking issue|Documentation coverage check failed|Exit-WithCode|blocking issue\(s\)'
     }
 }