.NET: Add missing new .slnx solution file

[Saibamen]

Addendum to #17373 and e791422

[Copilot]

Pull request overview

This PR adds support for the new .slnx XML-based solution file format introduced by Microsoft for .NET projects. It's an addendum to PR #17373, which improved network ecosystem inference for language-specific repositories. The .slnx format is now included alongside the traditional .sln format in all documentation and configuration files that describe .NET project detection.

Changes:

• Updated .NET solution file detection to include both .sln and .slnx formats across configuration and documentation
• Cleaned up trailing whitespace in multiple documentation lines

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
.serena/project.yml	Updated comment to specify that C# language server requires either .sln or .slnx file
.github/aw/github-agentic-workflows.md	Added .slnx to network inference rule for .NET repositories; removed trailing whitespace from multiple lines
.github/aw/create-agentic-workflow.md	Added .slnx to two network inference guidance sections for detecting .NET projects; removed trailing whitespace from multiple lines

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[dsyme]

@Saibamen Thanks for the contribution!