core: fix return of draw_mermaid_png and change to not save image by default (#19950)

- **Description:** Improvement for #19599: fixing missing return of
graph.draw_mermaid_png and improve it to make the saving of the rendered
image optional

Co-authored-by: Angel Igareta <angel.igareta@klarna.com>

libs/core/langchain_core/runnables/graph.py

@@ -369,11 +369,11 @@ class Graph:
             start="#ffdfba", end="#baffc9", other="#fad7de"
         ),
         wrap_label_n_words: int = 9,
-        output_file_path: str = "graph.png",
+        output_file_path: Optional[str] = None,
         draw_method: MermaidDrawMethod = MermaidDrawMethod.API,
         background_color: str = "white",
         padding: int = 10,
-    ) -> None:
+    ) -> bytes:
         from langchain_core.runnables.graph_mermaid import draw_mermaid_png
 
         mermaid_syntax = self.draw_mermaid(
@@ -381,7 +381,7 @@ class Graph:
             node_colors=node_colors,
             wrap_label_n_words=wrap_label_n_words,
         )
-        draw_mermaid_png(
+        return draw_mermaid_png(
             mermaid_syntax=mermaid_syntax,
             output_file_path=output_file_path,
             draw_method=draw_method,