What happened?

Describe the bug
When a team state with save_state a serialization exception is raised and the state is truncated at the first datetime.

To Reproduce
A Swarm team was created, with only one assistant agent:

# Assume existing and working model_client and project_search
project_agent = AssistantAgent(
    name="ProjectSearchAgent",
    model_client=model_client,
    tools=[project_search],
    system_message="You are a project search agent. Search for information and respond with JSON: {\"response\": \"your answer\", \"terminate\": \"TERMINATE\"}",
    output_content_type_format="json_object"
)

# Team setup
termination = TextMentionTermination("TERMINATE")
team = Swarm([project_agent], termination_condition=termination)

async def main():
    result = await team.run(task="What is a Montague?")
    
    team_state = await team.save_state()

    with open(state_file, "w") as f:
        json.dump(team_state, f) # This throws "Object of type datetime is not JSON serializable"

Expected behavior
The state should be dumped without errors, as per (documentation)[https://microsoft.github.io/autogen/stable//user-guide/agentchat-user-guide/tutorial/state.html#persisting-state-file-or-database].

Workaround
A good workaround is to jsonpickle the state. It should be documented, though

Which packages was the bug in?

Python AgentChat (autogen-agentchat>=0.4.0)

AutoGen library version.

Python 0.6.4

Other library version.

autogen-ext = {extras = ["anthropic"], version = "^0.6.2"}

Model used

eu.anthropic.claude-sonnet-4-20250514-v1:

Model provider

AWS Bedrock

Other model provider

No response

Python version

3.13

.NET version

None

Operating system

MacOS