feat: add get_thread() method to BaseGroupChat

[yahyasaqban-lab]

Why

For BaseGroupChat, there is currently no public method to retrieve all messages exchanged so far in a session. Users have to track messages externally. This PR adds a simple get_thread() method.

Changes

New Events (_events.py)

• GroupChatGetThread — request event for retrieving the current message thread
• GroupChatGetThreadResponse — response event containing the list of messages

BaseGroupChatManager (_base_group_chat_manager.py)

• Added handle_get_thread RPC handler that returns the current _message_thread as a GroupChatGetThreadResponse

BaseGroupChat (_base_group_chat.py)

• Added get_thread() public async method that sends a GroupChatGetThread request to the group chat manager and returns the message list

Usage

result = await team.run(task="Count from 1 to 10, respond one at a time.")
thread = await team.get_thread()
print(f"Total messages: {len(thread)}")
for msg in thread:
    print(f"{msg.source}: {msg.content}")

Fixes #6085