fix(core): resolve mermaid node id collisions when special chars are used

[casparb]

Description

• Replace the Mermaid graph node label escaping logic (_escape_node_label) with _to_safe_id, which converts a string into a unique, Mermaid-compatible node id. Ensures nodes with special characters always render correctly.

Before

• Invalid characters (e.g. 开) replaced with _. Causes collisions between nodes with names that are the same length and contain all non-safe characters:

_escape_node_label("开") # '_'
_escape_node_label("始") # '_'  same as above, but different character passed in. not a unique mapping.

After

_to_safe_id("开") # \5f00
_to_safe_id("始") # \59cb  unique!

Tests

• Rename test_graph_mermaid_escape_node_label() to test_graph_mermaid_to_safe_id() and update function logic to use _to_safe_id
• Add test_graph_mermaid_special_chars()

Issue

Fixes langchain-ai/langgraph#6036

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
langchain	Ignored	Preview		Sep 11, 2025 9:05pm

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #32857 will not alter performance

_{Comparing caspar/mermaid-special-char (94dec3d) with master (9cc8538)}

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Summary

✅ 13 untouched

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32857 will not alter performance

_{Comparing caspar/mermaid-special-char (94dec3d) with master (9cc8538)}

Summary

✅ 14 untouched

[casparb]

mermaid node ids can contain _, -, and \. Kept _ and - so slug is more human-readable. Use \ to escape code point representations.

[casparb]

fyi this was Parallel<as_list,as_str>Input and the previous function implementation replaced all nonascii with _. That's why it looks cleaner. But not the approach we want.

[sydney-runkle]

Nice!! Thanks for iterating