Respect -WhatIf in PowerShell package commands (#6588)

## 📖 Description

Fixes #4622.

Package commands run their work on an MTA thread. The existing
`PowerShellCmdlet.ShouldProcess` implementation returned `true` from
that thread instead of evaluating PowerShell's `PSCmdlet.ShouldProcess`,
so `-WhatIf` could permit real package operations.

Dispatch the decision through the existing PowerShell-thread bridge and
return the actual `ShouldProcess` result. Add regression coverage for
install, update, uninstall, MSIX repair, and export. Add a release note.

## 🔗 References

Fixes #4622

Author: huong8373tt-beep

.github/actions/spelling/allow.txt

@@ -450,3 +450,4 @@ Xlang
 XResource
 XTOKEN
 xunit
+whatif

doc/release-notes/1.31/6588-powershell-whatif.md

@@ -0,0 +1,5 @@
+## Bug Fixes
+
+### Minor Bug Fixes
+
+* Fixed PowerShell package commands ignoring `-WhatIf` and performing package operations during preview.

src/PowerShell/CommonFiles/PowerShellCmdlet.cs

@@ -502,13 +502,12 @@ internal void SetVariable(string variableName, object value)
         /// <returns>If the operation should continue.</returns>
         internal bool ShouldProcess(string target)
         {
-            // If not on the main thread just continue.
-            if (this.pwshThread != Thread.CurrentThread)
+            bool shouldProcess = false;
+            this.ExecuteInPowerShellThread(() =>
             {
-                return true;
-            }
-
-            return this.psCmdlet.ShouldProcess(target);
+                shouldProcess = this.psCmdlet.ShouldProcess(target);
+            });
+            return shouldProcess;
         }
 
         private void Complete()

src/PowerShell/tests/Microsoft.WinGet.Client.Tests.ps1

@@ -298,11 +298,34 @@ Describe 'Install|Update|Uninstall-WinGetPackage' {
         }
     }
 
+    It 'Does not install with WhatIf' {
+        $result = Install-WinGetPackage -Id AppInstallerTest.TestExeInstaller -Version '1.0.0.0' -WhatIf
+
+        $result | Should -BeNullOrEmpty
+        (Get-WinGetPackage -Id AppInstallerTest.TestExeInstaller -ErrorAction SilentlyContinue) | Should -BeNullOrEmpty
+    }
+
     It 'Install by Id' {
         $result = Install-WinGetPackage -Id AppInstallerTest.TestExeInstaller -Version '1.0.0.0'
         Validate-WinGetPackageOperationResult $result $expectedExeInstallerResult 'install'
     }
 
+    It 'Does not update with WhatIf' {
+        $installedVersion = (Get-WinGetPackage -Id AppInstallerTest.TestExeInstaller).InstalledVersion
+        $result = Update-WinGetPackage -Id AppInstallerTest.TestExeInstaller -WhatIf
+
+        $result | Should -BeNullOrEmpty
+        (Get-WinGetPackage -Id AppInstallerTest.TestExeInstaller).InstalledVersion | Should -Be $installedVersion
+    }
+
+    It 'Does not uninstall with WhatIf' {
+        $installedVersion = (Get-WinGetPackage -Id AppInstallerTest.TestExeInstaller).InstalledVersion
+        $result = Uninstall-WinGetPackage -Id AppInstallerTest.TestExeInstaller -WhatIf
+
+        $result | Should -BeNullOrEmpty
+        (Get-WinGetPackage -Id AppInstallerTest.TestExeInstaller).InstalledVersion | Should -Be $installedVersion
+    }
+
     It 'Install by exact Name and Version' {
         $result = Install-WinGetPackage -Name TestPortableExe -Version '2.0.0.0' -MatchOption Equals
         Validate-WinGetPackageOperationResult $result $expectedPortableInstallerResult 'install'
@@ -369,6 +392,14 @@ Describe 'Install|Repair|Uninstall-WinGetPackage' {
             Validate-WinGetPackageOperationResult $result $expectedResult 'install'
         }
 
+        It 'Does not repair MSIX with WhatIf' {
+            $installedVersion = (Get-WinGetPackage -Id AppInstallerTest.TestMsixInstaller).InstalledVersion
+            $result = Repair-WinGetPackage -Id AppInstallerTest.TestMsixInstaller -WhatIf
+
+            $result | Should -BeNullOrEmpty
+            (Get-WinGetPackage -Id AppInstallerTest.TestMsixInstaller).InstalledVersion | Should -Be $installedVersion
+        }
+
         It 'Repair MSIX By Id' {
             $result = Repair-WinGetPackage -Id AppInstallerTest.TestMsixInstaller
             Validate-WinGetPackageOperationResult $result $expectedResult 'repair'
@@ -594,6 +625,14 @@ Describe 'Export-WinGetPackage' {
         AddTestSource
     }
 
+    It 'Does not download with WhatIf' {
+        $testDirectory = GetRandomTestDirectory
+        $result = Export-WinGetPackage -Id AppInstallerTest.TestExeInstaller -Version '1.0.0.0' -DownloadDirectory $testDirectory -WhatIf
+
+        $result | Should -BeNullOrEmpty
+        Test-Path -Path $testDirectory | Should -Be $false
+    }
+
     It 'Download by Id' {
         $testDirectory = GetRandomTestDirectory
         $result = Export-WinGetPackage -Id AppInstallerTest.TestExeInstaller -Version '1.0.0.0' -DownloadDirectory $testDirectory