add support for pydantic state for the swarm state

[eyurtsev]

• Adds support for using pydantic base model for the swarm state.

[Copilot]

Pull Request Overview

This PR adds support for using Pydantic BaseModel as the state schema for swarm functionality, extending beyond the existing dictionary-based state approach. The changes enable developers to define typed state schemas using Pydantic models while maintaining backward compatibility.

• Introduces a generic _get_field utility function to handle field access across different state types (dict, dataclass, Pydantic)
• Updates the handoff tool implementation to work with both dictionary and Pydantic-based state schemas
• Adds comprehensive test coverage for the new Pydantic state functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
langgraph_swarm/handoff.py	Adds _get_field utility and updates handoff tool to support multiple state types
tests/test_swarm.py	Adds test case demonstrating Pydantic state schema usage with swarm functionality

[Copilot]

[nitpick] The unpacking syntax [*_get_field(state, "messages"), tool_message] could be clearer. Consider using _get_field(state, "messages") + [tool_message] for better readability, especially since the original code used concatenation.

Suggested change

	"messages": [*_get_field(state, "messages"), tool_message],
	"messages": _get_field(state, "messages") + [tool_message],