Make slnx / csproj always use relative paths (#4019)

* Make slnx / csproj always use relative paths

---------

Co-authored-by: Kaydax <kaydax@kaydax.xyz>

engine/Sandbox.SolutionGenerator/NativeFileSystem.cs

@@ -0,0 +1,77 @@
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+
+namespace Sandbox.SolutionGenerator;
+
+/// <summary>
+/// Provides native file system operations with platform-specific implementations.
+/// </summary>
+internal static partial class NativeFileSystem
+{
+	[DllImport( "kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true )]
+	private static extern IntPtr CreateFileW( string lpFileName, uint dwDesiredAccess, uint dwShareMode, IntPtr lpSecurityAttributes, uint dwCreationDisposition, uint dwFlagsAndAttributes, IntPtr hTemplateFile );
+
+	[DllImport( "kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true )]
+	private static extern uint GetFinalPathNameByHandleW( IntPtr hFile, char[] lpszFilePath, uint cchFilePath, uint dwFlags );
+
+	[DllImport( "kernel32.dll", SetLastError = true )]
+	private static extern bool CloseHandle( IntPtr hObject );
+
+	private const uint FILE_FLAG_BACKUP_SEMANTICS = 0x02000000;
+	private const uint OPEN_EXISTING = 3;
+	private const uint FILE_SHARE_READ = 1;
+	private const uint FILE_SHARE_WRITE = 2;
+
+	/// <summary>
+	/// Gets the canonical path with proper casing for the given path.
+	/// On Windows, this resolves the true filesystem path including correct casing.
+	/// On Linux, this returns the path unchanged since the filesystem is case-sensitive.
+	/// </summary>
+	/// <param name="path">The path to canonicalize.</param>
+	/// <returns>The canonical path, or the original path if canonicalization fails.</returns>
+	internal static string GetCanonicalPath( string path )
+	{
+		if ( string.IsNullOrWhiteSpace( path ) || !Path.IsPathRooted( path ) )
+			return path;
+
+		if ( !OperatingSystem.IsWindows() )
+			return path;
+
+		try
+		{
+			var handle = CreateFileW( path, 0, FILE_SHARE_READ | FILE_SHARE_WRITE, IntPtr.Zero, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, IntPtr.Zero );
+			if ( handle == IntPtr.Zero || handle == new IntPtr( -1 ) )
+				return path;
+
+			try
+			{
+				var buffer = new char[512];
+				var len = GetFinalPathNameByHandleW( handle, buffer, (uint)buffer.Length, 0 );
+				if ( len > 0 && len < buffer.Length )
+				{
+					var finalPath = new string( buffer, 0, (int)len );
+					// Remove the \\?\ prefix added by Windows API
+					if ( finalPath.StartsWith( @"\\?\" ) )
+					{
+						finalPath = finalPath.Substring( 4 );
+					}
+					return finalPath;
+				}
+				else
+				{
+					return path;
+				}
+			}
+			finally
+			{
+				CloseHandle( handle );
+			}
+		}
+		catch
+		{
+			// Ignore errors and return original path
+			return path;
+		}
+	}
+}