core[minor]: handle boolean data in draw_mermaid (#23135)

This change should address graph rendering issues for edges with boolean
data

Example from langgraph:

```python
from typing import Annotated, TypedDict

from langchain_core.messages import AnyMessage
from langgraph.graph import END, START, StateGraph
from langgraph.graph.message import add_messages


class State(TypedDict):
    messages: Annotated[list[AnyMessage], add_messages]


def branch(state: State) -> bool:
    return 1 + 1 == 3


graph_builder = StateGraph(State)
graph_builder.add_node("foo", lambda state: {"messages": [("ai", "foo")]})
graph_builder.add_node("bar", lambda state: {"messages": [("ai", "bar")]})

graph_builder.add_conditional_edges(
    START,
    branch,
    path_map={True: "foo", False: "bar"},
    then=END,
)

app = graph_builder.compile()
print(app.get_graph().draw_mermaid())
```

Previous behavior:

```python
AttributeError: 'bool' object has no attribute 'split'
```

Current behavior:

```python
%%{init: {'flowchart': {'curve': 'linear'}}}%%
graph TD;
	__start__[__start__]:::startclass;
	__end__[__end__]:::endclass;
	foo([foo]):::otherclass;
	bar([bar]):::otherclass;
	__start__ -. ('a',) .-> foo;
	foo --> __end__;
	__start__ -. ('b',) .-> bar;
	bar --> __end__;
	classDef startclass fill:#ffdfba;
	classDef endclass fill:#baffc9;
	classDef otherclass fill:#fad7de;
```

libs/core/langchain_core/runnables/graph.py

@@ -11,6 +11,7 @@ from typing import (
     List,
     NamedTuple,
     Optional,
+    Protocol,
     Tuple,
     Type,
     TypedDict,
@@ -25,6 +26,11 @@ if TYPE_CHECKING:
     from langchain_core.runnables.base import Runnable as RunnableType
 
 
+class Stringifiable(Protocol):
+    def __str__(self) -> str:
+        ...
+
+
 class LabelsDict(TypedDict):
     """Dictionary of labels for nodes and edges in a graph."""
 
@@ -55,7 +61,7 @@ class Edge(NamedTuple):
 
     source: str
     target: str
-    data: Optional[str] = None
+    data: Optional[Stringifiable] = None
     conditional: bool = False
 
 
@@ -253,7 +259,7 @@ class Graph:
         self,
         source: Node,
         target: Node,
-        data: Optional[str] = None,
+        data: Optional[Stringifiable] = None,
         conditional: bool = False,
     ) -> Edge:
         """Add an edge to the graph and return it."""

libs/core/langchain_core/runnables/graph_mermaid.py

@@ -81,7 +81,7 @@ def draw_mermaid(
         # Add BR every wrap_label_n_words words
         if edge.data is not None:
             edge_data = edge.data
-            words = edge_data.split()  # Split the string into words
+            words = str(edge_data).split()  # Split the string into words
             # Group words into chunks of wrap_label_n_words size
             if len(words) > wrap_label_n_words:
                 edge_data = "<br>".join(

libs/core/langchain_core/runnables/graph_png.py

@@ -160,8 +160,8 @@ class PngDrawer:
             self.add_node(viz, node)
 
     def add_edges(self, viz: Any, graph: Graph) -> None:
-        for start, end, label, cond in graph.edges:
-            self.add_edge(viz, start, end, label, cond)
+        for start, end, data, cond in graph.edges:
+            self.add_edge(viz, start, end, str(data), cond)
 
     def update_styles(self, viz: Any, graph: Graph) -> None:
         if first := graph.first_node():