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/Modules/Microsoft.PowerShell.Security/AclCmdlets.Tests.ps1

@@ -3,6 +3,7 @@
 Describe "Acl cmdlets are available and operate properly" -Tag CI {
     Context "Windows ACL test" {
         BeforeAll {
+            $originalDefaultParameterValues = $PSDefaultParameterValues.Clone()
             $PSDefaultParameterValues["It:Skip"] = -not $IsWindows
         }
 
@@ -103,7 +104,7 @@ Describe "Acl cmdlets are available and operate properly" -Tag CI {
         }
 
         AfterAll {
-            $PSDefaultParameterValues.Remove("It:Skip")
+            $global:PSDefaultParameterValues = $originalDefaultParameterValues
         }
     }
 }