feat: Add --what-if for Microsoft.Windows/UpdateList

[Gijsreyn]

PR Summary

This pull request adds --what-if support for Microsoft.Windows/UpdateList.

PR Context

Partially addresses #1529

[Copilot]

Pull request overview

This PR adds --what-if support for the built-in Microsoft.Windows/UpdateList resource by wiring the manifest to pass the what-if flag, projecting “after” state during what-if execution, and exposing what-if messages via _metadata.

Changes:

• Updated the Microsoft.Windows/UpdateList resource manifest to declare what-if behavior for set (whatIfArg/whatIfReturns) and align with v3 manifest fields (implementsPretest).
• Added projected what-if behavior in the Windows Update set implementation, returning _metadata.whatIf messages instead of mutating state.
• Added Pester tests covering what-if behavior for installing and already-installed updates.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
resources/WindowsUpdate/windowsupdate.dsc.resource.json	Enables what-if for set and adds _metadata.whatIf schema support.
resources/WindowsUpdate/tests/windowsupdate_whatif.tests.ps1	Adds what-if integration tests for Microsoft.Windows/UpdateList.
resources/WindowsUpdate/src/windows_update/types.rs	Introduces _metadata Rust types and includes them in UpdateInfo.
resources/WindowsUpdate/src/windows_update/set.rs	Implements what-if projection for update installation with localized what-if messages.
resources/WindowsUpdate/src/windows_update/export.rs	Ensures UpdateInfo construction includes metadata: None.
resources/WindowsUpdate/src/main.rs	Threads what-if detection into handle_set.
resources/WindowsUpdate/locales/en-us.toml	Adds localized what-if message template for update installation.