Support PSObject wrapped values in ArgumentToEncodingTransformationAttribute (#24555)

src/System.Management.Automation/utils/EncodingUtils.cs

@@ -115,6 +115,8 @@ namespace System.Management.Automation
     {
         public override object Transform(EngineIntrinsics engineIntrinsics, object inputData)
         {
+            inputData = PSObject.Base(inputData);
+
             switch (inputData)
             {
                 case string stringName:

test/powershell/engine/Basic/Encoding.Tests.ps1

@@ -89,6 +89,8 @@ Describe "File encoding tests" -Tag CI {
         It "Parameter 'Encoding' should accept '<encoding>'" -TestCases @(
             @{ encoding = 1251 }
             @{ encoding = "windows-1251" }
+            # Piping the string creates a PSObject boxed value that we are testing.
+            @{ encoding = ("windows-1251" | Write-Output) }
         ) {
             param ( $encoding )
             $testFile = "${TESTDRIVE}/fileEncoding-$($encoding).txt"