Respect -ListAvailable:$false in Get-TimeZone (#26463)

src/Microsoft.PowerShell.Commands.Management/commands/management/TimeZoneCommands.cs

@@ -54,7 +54,7 @@ namespace Microsoft.PowerShell.Commands
             // make sure we've got the latest time zone settings
             TimeZoneInfo.ClearCachedData();
 
-            if (this.ParameterSetName.Equals("ListAvailable", StringComparison.OrdinalIgnoreCase))
+            if (ListAvailable)
             {
                 // output the list of all available time zones
                 WriteObject(TimeZoneInfo.GetSystemTimeZones(), true);

test/powershell/Modules/Microsoft.PowerShell.Management/TimeZone.Tests.ps1

@@ -71,6 +71,12 @@ Describe "Get-Timezone test cases" -Tags "CI" {
         $oneExpectedOffset | Should -BeIn $observedIdList
     }
 
+    It "Call with -ListAvailable:`$false returns current TimeZoneInfo (not list)" {
+        $result = Get-TimeZone -ListAvailable:$false
+        $result | Should -BeOfType TimeZoneInfo
+        $result.Id | Should -Be ([System.TimeZoneInfo]::Local).Id
+    }
+
     It "Call Get-TimeZone using ID param and single item" {
         $selectedTZ = $TimeZonesAvailable[0]
         (Get-TimeZone -Id $selectedTZ.Id).Id | Should -Be $selectedTZ.Id