fix: Selecting multiple lines in RTL textbox

[AhmedZero]

What does the pull request do?

fixes #19046, This PR modifies the text layout system to properly handle Right-to-Left (RTL) languages while maintaining existing Left-to-Right (LTR) support.

What is the current behavior?

What is the updated/expected behavior with this PR?

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

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

fixes #19046

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0057191-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[cla-avalonia]

• All contributors have signed the CLA.

[AhmedZero]

@cla-avalonia agree

[MrJul]

Thank you for your contribution!
This needs some unit tests to better understand the fixed issue (and make sure we won't regress it in the future).

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0057219-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0057223-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Gillibald]

Please make the new test a theory and add some RTL text and mixed text to verify this is working with bidi text.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0057261-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]