fix(SelectableTextBlock): fix SelectableTextBlock selection for centered and right-aligned text

[zrt2399]

fix #21368

What does the pull request do?

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

SelectableTextBlock was unable to select the full text content when TextAlignment was set to Center or Right.
The issue was caused by pointer hit-testing during drag selection clamping the X coordinate to TextLayout.WidthIncludingTrailingWhitespace. For centered and right-aligned text, the rendered text can start later within the available layout width, so clamping to the text width prevented the selection logic from reaching the trailing characters.
This change updates the drag-selection hit-testing logic to clamp against the available layout width when appropriate, allowing selection to extend to the visual end of the text for non-left-aligned content.
Also added regression tests covering drag selection with Center and Right text alignment, and normalized the sample XAML self-closing tag formatting to keep build validation clean.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

fix #21368

[avaloniaui-bot]

You can test this PR using the following package version. 12.1.999-cibuild0065457-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]