fix: change node names not supported in memaid rendering

[JadenKim-dev]

When rendering a graph using mermaid, if the node name contains _node, such as answer_node, the following error occurs and rendering fails.

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[6], line 22
     19 graph = graph.compile()
     21 # View
---> 22 display(Image(graph.get_graph().draw_mermaid_png()))

File ~/project/langchain-academy/lc-academy-env/lib/python3.13/site-packages/langchain_core/runnables/graph.py:704, in Graph.draw_mermaid_png(self, curve_style, node_colors, wrap_label_n_words, output_file_path, draw_method, background_color, padding, max_retries, retry_delay, frontmatter_config, base_url)
    694 from langchain_core.runnables.graph_mermaid import (  # noqa: PLC0415
    695     draw_mermaid_png,
    696 )
    698 mermaid_syntax = self.draw_mermaid(
    699     curve_style=curve_style,
    700     node_colors=node_colors,
    701     wrap_label_n_words=wrap_label_n_words,
    702     frontmatter_config=frontmatter_config,
    703 )
--> 704 return draw_mermaid_png(
    705     mermaid_syntax=mermaid_syntax,
    706     output_file_path=output_file_path,
    707     draw_method=draw_method,
    708     background_color=background_color,
    709     padding=padding,
    710     max_retries=max_retries,
    711     retry_delay=retry_delay,
    712     base_url=base_url,
    713 )

File ~/project/langchain-academy/lc-academy-env/lib/python3.13/site-packages/langchain_core/runnables/graph_mermaid.py:322, in draw_mermaid_png(mermaid_syntax, output_file_path, draw_method, background_color, padding, max_retries, retry_delay, base_url)
    316     img_bytes = asyncio.run(
    317         _render_mermaid_using_pyppeteer(
    318             mermaid_syntax, output_file_path, background_color, padding
    319         )
    320     )
    321 elif draw_method == MermaidDrawMethod.API:
--> 322     img_bytes = _render_mermaid_using_api(
    323         mermaid_syntax,
    324         output_file_path=output_file_path,
    325         background_color=background_color,
    326         max_retries=max_retries,
    327         retry_delay=retry_delay,
    328         base_url=base_url,
    329     )
    330 else:
    331     supported_methods = ", ".join([m.value for m in MermaidDrawMethod])

File ~/project/langchain-academy/lc-academy-env/lib/python3.13/site-packages/langchain_core/runnables/graph_mermaid.py:480, in _render_mermaid_using_api(mermaid_syntax, output_file_path, background_color, file_type, max_retries, retry_delay, base_url)
    475     # For other status codes, fail immediately
    476     msg = (
    477         f"Failed to reach {base_url} API while trying to render "
    478         f"your graph. Status code: {response.status_code}.\n\n"
    479     ) + error_msg_suffix
--> 480     raise ValueError(msg)
    482 except (requests.RequestException, requests.Timeout) as e:
    483     if attempt < max_retries:
    484         # Exponential backoff with jitter

ValueError: Failed to reach https://mermaid.ink/ API while trying to render your graph. Status code: 204.

To resolve this issue:
1. Check your internet connection and try again
2. Try with higher retry settings: `draw_mermaid_png(..., max_retries=5, retry_delay=2.0)`
3. Use the Pyppeteer rendering method which will render your graph locally in a browser: `draw_mermaid_png(..., draw_method=MermaidDrawMethod.PYPPETEER)`

I modified it to render successfully by changing the node names to answer_node -> answer, thinking_node -> thinking.