Fix absolute path checking on windows

The fragile path matching no longer works with dotnet 8, causing Windows users to lose collection items because the item path is incorrectly validated, then the server will remove all items due to file not found.

Signed-off-by: gnattu <gnattuoc@me.com>

Emby.Server.Implementations/IO/ManagedFileSystem.cs

@@ -80,12 +80,14 @@ namespace Emby.Server.Implementations.IO
         public virtual string MakeAbsolutePath(string folderPath, string filePath)
         {
             // path is actually a stream
-            if (string.IsNullOrWhiteSpace(filePath) || filePath.Contains("://", StringComparison.Ordinal))
+            if (string.IsNullOrWhiteSpace(filePath))
             {
                 return filePath;
             }
 
-            if (filePath.Length > 3 && filePath[1] == ':' && filePath[2] == '/')
+            var isAbsolutePath = Path.IsPathRooted(filePath) && (!OperatingSystem.IsWindows() || filePath[0] != '\\');
+
+            if (isAbsolutePath)
             {
                 // absolute local path
                 return filePath;
@@ -97,17 +99,10 @@ namespace Emby.Server.Implementations.IO
                 return filePath;
             }
 
-            var firstChar = filePath[0];
-            if (firstChar == '/')
-            {
-                // for this we don't really know
-                return filePath;
-            }
-
             var filePathSpan = filePath.AsSpan();
 
-            // relative path
-            if (firstChar == '\\')
+            // relative path on windows
+            if (filePath[0] == '\\')
             {
                 filePathSpan = filePathSpan.Slice(1);
             }