fix(core): resolve mermaid node id collisions when special chars are used (#32857)

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

**After**
```python
_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

libs/core/langchain_core/runnables/graph_mermaid.py

@@ -6,6 +6,7 @@ import asyncio
 import base64
 import random
 import re
+import string
 import time
 from dataclasses import asdict
 from pathlib import Path
@@ -148,7 +149,7 @@ def draw_mermaid(
                 + "</em></small>"
             )
         node_label = format_dict.get(key, format_dict[default_class_label]).format(
-            _escape_node_label(key), label
+            _to_safe_id(key), label
         )
         return f"{indent}{node_label}\n"
 
@@ -211,8 +212,7 @@ def draw_mermaid(
                 edge_label = " -.-> " if edge.conditional else " --> "
 
             mermaid_graph += (
-                f"\t{_escape_node_label(source)}{edge_label}"
-                f"{_escape_node_label(target)};\n"
+                f"\t{_to_safe_id(source)}{edge_label}{_to_safe_id(target)};\n"
             )
 
         # Recursively add nested subgraphs
@@ -256,9 +256,18 @@ def draw_mermaid(
     return mermaid_graph
 
 
-def _escape_node_label(node_label: str) -> str:
-    """Escapes the node label for Mermaid syntax."""
-    return re.sub(r"[^a-zA-Z-_0-9]", "_", node_label)
+def _to_safe_id(label: str) -> str:
+    """Convert a string into a Mermaid-compatible node id.
+
+    Keep [a-zA-Z0-9_-] characters unchanged.
+    Map every other character -> backslash + lowercase hex codepoint.
+
+    Result is guaranteed to be unique and Mermaid-compatible,
+    so nodes with special characters always render correctly.
+    """
+    allowed = string.ascii_letters + string.digits + "_-"
+    out = [ch if ch in allowed else "\\" + format(ord(ch), "x") for ch in label]
+    return "".join(out)
 
 
 def _generate_mermaid_graph_styles(node_colors: NodeStyles) -> str: