Document Pydantic Schema Usage

[eyurtsev]

Issue with current documentation:

from langgraph.graph import StateGraph
from langgraph.constants import START
from pydantic import BaseModel

class Foo(BaseModel):
    x: int
    y: str

def node(state: Foo):
    """This parent node will invoke the subgraph."""
    return state

builder = StateGraph(Foo)
builder.add_node("node", node)
builder.add_edge(START, "node")
graph = builder.compile()
result = graph.invoke(Foo(x=3, y="hello"))
Foo(**result)

Idea or request for content:

Figure out where to document current behavior + run time coercion

[hinthornw]

Another point here is that if you use BaseMessage without the add_messages reducer (or an equivalent), messages you send over the wire will be deserialized as just that.
instead should use AnyMessage.