[XSG] Enable XSG on HostApp

[StephaneDelcroix]

Description of Change

Enable XSG on HostApp and fix nullability issue

requires #32034

[Copilot]

Pull Request Overview

Enable XAML Source Generator (XSG) in the HostApp test project and adjust event handler signatures for nullable sender parameters.

• Add MauiXaml ItemGroup with Inflator="SourceGen" to Controls.TestCases.HostApp.csproj.
• Update several event handler signatures to accept nullable object? sender (nullability fix).

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Issue25724.xaml.cs	Updated event handler sender parameter to nullable.
Issue23868.xaml.cs	Updated multiple event handler sender parameters to nullable.
Issue20903.xaml.cs	Updated button click handler sender parameter to nullable.
Issue14825.xaml.cs	Updated capture button handler sender parameter to nullable.
Controls.TestCases.HostApp.csproj	Enabled XAML Source Generation for all XAML files in the HostApp project.