Add get_thread method to BaseGroupChatManager and BaseGroupChat (Fixes #6085)

[MUKKTINAADH (mukktinaadh)]

Summary

This PR adds a get_thread method to both BaseGroupChatManager and BaseGroupChat classes to support retrieving the current message thread from a group chat team.

Changes

1. BaseGroupChatManager.get_thread(): Added a new method that returns a copy of the current message thread (_message_thread). Returns a copy for thread safety.

2. BaseGroupChat.get_thread(): Added a public method that retrieves the message thread from the group chat manager via RPC. The method:

   • Checks if the team is initialized
   • Calls the get_thread RPC method on the group chat manager
   • Returns the message thread

Usage

# Get the current message thread from a group chat team
thread = await team.get_thread()
for message in thread:
    print(f'{message.source}: {message.content}')

Testing

Added basic verification that:

• BaseGroupChatManager.get_thread() returns a copy of the message thread
• The returned thread is a copy (modifications don't affect the original)
• BaseGroupChat.get_thread() calls the manager via RPC

Fixes #6085

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