Fix $PSDefaultParameterValues leak causing tests to skip unexpectedly (#26602)

This fixes test isolation issues where six test files were leaking $PSDefaultParameterValues["It:Skip"] settings to subsequent tests, causing widespread test skipping on Linux. The fixes address two distinct patterns: using .Remove() which doesn't restore the original state (4 files), and imbalanced Push-DefaultParameterValueStack/Pop-DefaultParameterValueStack calls (2 files).

test/powershell/engine/ETS/CimAdapter.Tests.ps1

@@ -3,6 +3,8 @@
 
 Describe "CIM Objects are adapted properly" -Tag @("CI") {
     BeforeAll {
+        $originalDefaultParameterValues = $PSDefaultParameterValues.Clone()
+        
         function getIndex
         {
             param([string[]]$strings,[string]$pattern)
@@ -32,7 +34,7 @@ Describe "CIM Objects are adapted properly" -Tag @("CI") {
         }
     }
     AfterAll {
-        $PSDefaultParameterValues.Remove("it:pending")
+        $global:PSDefaultParameterValues = $originalDefaultParameterValues
     }
 
     It "Namespace-qualified Win32_Process is present" -Skip:(!$IsWindows) {

test/powershell/engine/Remoting/RemoteSession.Basic.Tests.ps1

@@ -83,9 +83,7 @@ Describe "JEA session Transcript script test" -Tag @("Feature", 'RequireAdminOnW
     AfterAll {
         if ($skipTest) {
             Pop-DefaultParameterValueStack
-            return
         }
-        Pop-DefaultParameterValueStack
     }
 
     It "Configuration name should be in the transcript header" {

test/powershell/engine/Remoting/SessionOption.Tests.ps1

@@ -1,6 +1,7 @@
 # Copyright (c) Microsoft Corporation.
 # Licensed under the MIT License.
 try {
+    $originalDefaultParameterValues = $PSDefaultParameterValues.Clone()
     if ( ! $IsWindows ) {
         $PSDefaultParameterValues['it:skip'] = $true
     }
@@ -52,5 +53,5 @@ try {
     }
 }
 finally {
-    $PSDefaultParameterValues.remove("it:skip")
+    $global:PSDefaultParameterValues = $originalDefaultParameterValues
 }

test/powershell/engine/Security/FileSignature.Tests.ps1

@@ -89,6 +89,7 @@ Describe "Windows file content signatures" -Tags @('Feature', 'RequireAdminOnWin
 
     AfterAll {
         if ($shouldSkip) {
+            Pop-DefaultParameterValueStack
             return
         }