Use Field(default_factory=...) on State mutables

Match the convention in docs/concepts/state-and-reducers.md.
Pattern 2 and pattern 3 State models had list/dict fields with
mutable defaults (=[], ={}); convert to Field(default_factory=...)
and import Field from pydantic.

Patterns 1 and 4 have no mutable defaults so they're unchanged.

Author: chris-colinsky

docs/patterns/session-as-checkpoint-resume.md

@@ -26,14 +26,15 @@ across resume).
 
 ```python
 from typing import Annotated
+from pydantic import Field
 from openarmature.checkpoint import SQLiteCheckpointer
 from openarmature.graph import END, GraphBuilder, State, append, merge
 from openarmature.llm import Message
 
 
 class SessionState(State):
-    messages: Annotated[list[Message], append] = []
-    facts: Annotated[dict[str, str], merge] = {}
+    messages: Annotated[list[Message], append] = Field(default_factory=list)
+    facts: Annotated[dict[str, str], merge] = Field(default_factory=dict)
     last_user_input: str = ""
 
 

docs/patterns/tool-dispatch-as-node.md

@@ -21,12 +21,13 @@ on top of [`Tool`, `ToolCall`, `ToolMessage`](../concepts/llms.md).
 ```python
 import json
 from typing import Annotated
+from pydantic import Field
 from openarmature.graph import END, EndSentinel, GraphBuilder, State, append
 from openarmature.llm import AssistantMessage, Message, Tool, ToolMessage
 
 
 class AgentState(State):
-    messages: Annotated[list[Message], append] = []
+    messages: Annotated[list[Message], append] = Field(default_factory=list)
     turn: int = 0