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.Tests/TestData/GeneratedSources/Generic(Of T)_ScriptPath.generated.cs

@@ -1,5 +1,5 @@
 using Godot;
 [ScriptPathAttribute("res://Generic.cs")]
-partial class Generic
+partial class Generic<T>
 {
 }

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/GeneratedSources/NamespaceA.SameName_ScriptPath.generated.cs

@@ -0,0 +1,9 @@
+using Godot;
+namespace NamespaceA {
+
+[ScriptPathAttribute("res://SameName.cs")]
+partial class SameName
+{
+}
+
+}

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/Sources/Generic.GD0003.cs

@@ -0,0 +1,18 @@
+using Godot;
+
+partial class Generic<T> : GodotObject
+{
+    private int _field;
+}
+
+// Generic again but different generic parameters
+partial class {|GD0003:Generic|}<T, R> : GodotObject
+{
+    private int _field;
+}
+
+// Generic again but without generic parameters
+partial class {|GD0003:Generic|} : GodotObject
+{
+    private int _field;
+}

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/Sources/Generic.cs

@@ -4,15 +4,3 @@ 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
-{
-    private int _field;
-}

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/Sources/SameName.GD0003.cs

@@ -0,0 +1,18 @@
+using Godot;
+
+namespace NamespaceA
+{
+    partial class SameName : GodotObject
+    {
+        private int _field;
+    }
+}
+
+// SameName again but different namespace
+namespace NamespaceB
+{
+    partial class {|GD0003:SameName|} : GodotObject
+    {
+        private int _field;
+    }
+}