Add ContentDialog item template to the VSIX

[lauren-ciha]

Resolves #3723 and #5941

The ContentDialog item template was present since #6407 for the dotnet templates. To add them to the VSIX templates, they needed unique strings and $parameter$ syntax instead of __WINUI_CONTENT_DIALOG__, the latter of which only works with the dotnet template engine. The $parameter$ syntax works in both.

Validation:

• Fetch and build the branch with .\dev\Templates\VSIX\build-local-VSIX-packagebuild-install-localdev-vsix.ps1.
• For each C# templates in Visual Studio:
  • Created a project with the template
  • Opened the New Item menu (Ctrl+Shift+A)
  • Selected the ContentDialog
  • Added a button to the main page
  • Built and deployed with F5
  • Confirmed that the ContentDialog appeared on the button click
• For the Class library template, I confirmed that the solution built with the content dialog included
• Confirmed there were no regressions on the dotnet templates with .\dev\Templates\Dotnet\Test-DotnetNewTemplates.ps1 -WindowsAppSDkVersion "2.2.0" -dotnetsdkversion "*"

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[lauren-ciha]

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[Copilot]

Pull request overview

This pull request adds the WinUI 3 ContentDialog item template to the Visual Studio VSIX templates, aligning the VSIX and dotnet new item template sources by using VS-compatible $parameter$ replacements and adding the necessary VSIX string resources.

Changes:

• Add new VSIX string resources for the ContentDialog item template (including an experimental name variant).
• Update the ContentDialog .vstemplate to reference the new string IDs.
• Update the shared ContentDialog item template source (.xaml / .xaml.cs) to use $rootnamespace$ / $safeitemname$ and concrete WinUI types so it works for both VSIX and dotnet new.
• Adjust experimental VSIX DisplayName generation to avoid exceeding the manifest’s 50-character limit.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dev/Templates/VSIX/Extension/Directory.Build.targets	Updates experimental VSIX DisplayName generation logic (length constraint handling).
dev/Templates/VSIX/Extension/Cs/Common/VSPackage.resx	Adds new string resources (name/experimental name/description) for the ContentDialog template.
dev/Templates/Source/ItemTemplates/Neutral/CSharp/ContentDialog/WinUI.Neutral.Cs.ContentDialog.vstemplate	Switches Name/Description IDs to the newly added string resources.
dev/Templates/Source/ItemTemplates/Neutral/CSharp/ContentDialog/ContentDialog.xaml.cs	Replaces dotnet-template-only tokens with VSIX-compatible parameters and concrete WinUI types.
dev/Templates/Source/ItemTemplates/Neutral/CSharp/ContentDialog/ContentDialog.xaml	Replaces dotnet-template-only tokens with VSIX-compatible parameters and concrete WinUI markup.