Multi project support for the C# parser

[HorvathDaniel15]

Resolves #823

[Copilot]

Pull request overview

This PR extends the C# parser to accept richer inputs (solutions/projects) and to better handle multiple inputs, enabling “multi-project” parsing via Roslyn/MSBuild rather than only directory-based discovery.

Changes:

• Add input validation and safer log parsing in the C++ wrapper (avoid empty-input and empty-line crashes; log non file-status output).
• Update the C# parser to support .sln/.slnx and .csproj inputs using MSBuildWorkspace, with a fallback path that extracts project paths from solution files.
• Add Roslyn MSBuild workspace dependency and ignore .DS_Store.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 7 comments.

File	Description
plugins/csharp/parser/src/csharpparser.cpp	Validates inputs and hardens parsing of the C# parser output stream.
plugins/csharp/parser/src_csharp/Program.cs	Implements solution/project loading via MSBuildWorkspace and multi-project processing with parallel document parsing.
plugins/csharp/parser/src_csharp/CSharpParser.csproj	Adds Microsoft.CodeAnalysis.Workspaces.MSBuild to support solution/project loading.
.gitignore	Ignores macOS .DS_Store.

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