fix(robot): do not override the source on a model if there is already a source field

Newer Robot Framework versions already set the source field on File AST blocks, so overriding it here is unnecessary.

fixes #588

Author: d-biehl

packages/robot/src/robotcode/robot/diagnostics/document_cache_helper.py

@@ -319,7 +319,8 @@ def get_tokens(source: str, data_only: bool = False, lang: Any = None) -> Iterat
         else:
             model = _get_model(get_tokens, document.uri.to_path(), False, None)
 
-        model.source = str(document.uri.to_path())
+        if not hasattr(model, "source"):
+            model.source = str(document.uri.to_path())
         model.model_type = document_type
 
         return cast(ast.AST, model)