Fix #2585: Prevent $WhatIfPreference from breaking Pester (#2730)

Set $WhatIfPreference = $false at the start of Invoke-Pester's begin
block. When a caller sets $WhatIfPreference = $true, it leaks into
Pester's internal commands (New-Item, Remove-Item, etc.) and causes
test execution to fail with cryptic errors or excessive 'What if:'
messages. The local assignment scopes it to Invoke-Pester and its
children without affecting the caller's preference.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nohwnd

src/Main.ps1

@@ -454,6 +454,10 @@ function Invoke-Pester {
         [PesterConfiguration] $Configuration
     )
     begin {
+        # Prevent $WhatIfPreference from leaking into Pester internals (#2585).
+        # When the caller sets $WhatIfPreference = $true, it propagates to child
+        # scopes and breaks commands that Pester relies on (New-Item, Remove-Item, etc.).
+        $WhatIfPreference = $false
         $start = [DateTime]::Now
         # this will inherit to child scopes and allow Describe / Context to run directly from a file or command line
         $invokedViaInvokePester = $true

tst/Pester.RSpec.InNewProcess.ts.ps1

@@ -391,4 +391,30 @@ i -PassThru:$PassThru {
             }
         }
     }
+
+    b '$WhatIfPreference does not break Pester' {
+        t 'Invoke-Pester succeeds when $WhatIfPreference is $true' {
+                $sb = {
+                    $WhatIfPreference = $true
+
+                    $PesterPreference = [PesterConfiguration]::Default
+                    $PesterPreference.Output.Verbosity = 'None'
+
+                    $container = New-PesterContainer -ScriptBlock {
+                        Describe 'd1' {
+                            It 'i1' {
+                                1 | Should -Be 1
+                            }
+                        }
+                    }
+                    $r = Invoke-Pester -Container $container -PassThru
+                    if ($r.FailedCount -ne 0) { throw "Expected 0 failures, got $($r.FailedCount)" }
+                    if ($r.PassedCount -ne 1) { throw "Expected 1 passed, got $($r.PassedCount)" }
+                }
+
+                $output = Invoke-InNewProcess $sb
+                $whatIfLines = $output | Select-String -Pattern 'What if:'
+                @($whatIfLines).Count | Verify-Equal 0
+        }
+    }
 }