Updating doc to set correct expectations when using restrictive XAML mode in WPF

[dipeshmsft]

Summary

Reverting the security guidance added in #2261 for loading untrusted XAML, BAML and XPS packages. Most of the added changes are redundant as the expectations for the above scenario is already present in the doc. However, retaining the doc changes on restrictive XAML reader mode in WPF as the latest findings suggested that lack of documentation regarding this mode is setting wrong expectations for the developers.

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Internal previews

File	Preview link
dotnet-desktop-guide/wpf/security-wpf.md	dotnet-desktop-guide/wpf/security-wpf
dotnet-desktop-guide/xaml-services/security-considerations.md	dotnet-desktop-guide/xaml-services/security-considerations

[Copilot]

Pull request overview

This PR updates WPF and .NET XAML Services security documentation by reverting the broader “loading untrusted markup” guidance introduced in PR #2261, while keeping and emphasizing guidance that clarifies what WPF’s restrictive XAML reader mode does—and does not—guarantee.

Changes:

• Removed the recently added “untrusted XAML/binary XAML/restrictive reader” guidance block from the .NET XAML Services security considerations article.
• Removed the “Loading Untrusted XAML, BAML, and XPS Content” section from the WPF security article.
• Added a focused “Restrictive XAML reader mode” subsection under the existing loose XAML sandboxing section to set expectations about process/boundary behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
dotnet-desktop-guide/xaml-services/security-considerations.md	Reverts the PR #2261 guidance block and updates metadata date.
dotnet-desktop-guide/wpf/security-wpf.md	Removes the “untrusted markup” section, adds restrictive reader expectations, and refreshes the in-article section list.

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