C#: Various fixes to generic scripts

- Report a diagnostic when there are multiple classes that match the script file name in the same script since that will result in a duplicate path key in the bimap and it's not allowed.
- Fix InspectorPlugin to handle empty paths in case the project was built with a previous version of Godot that used empty paths for generic scripts.
- Add tests for the new diagnostic GD0003.

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Sample/Generic.cs

@@ -2,17 +2,6 @@
 
 namespace Godot.SourceGenerators.Sample
 {
-    partial class Generic<T> : GodotObject
-    {
-        private int _field;
-    }
-
-    // Generic again but different generic parameters
-    partial class Generic<T, R> : GodotObject
-    {
-        private int _field;
-    }
-
     // Generic again but without generic parameters
     partial class Generic : GodotObject
     {