feat: expose group chat message thread

[ynachiket]

Summary

• add a GroupChatGetThread RPC and GroupChatThread response for group chat managers
• expose BaseGroupChat.get_thread() as a public async snapshot API
• add regression coverage for empty state, resume, copy isolation, and reset behavior

Closes #6085

Validation

• /tmp/autogen-agentchat-6085-venv/bin/python -m pytest python/packages/autogen-agentchat/tests/test_group_chat.py -k get_thread -q
• /tmp/autogen-agentchat-6085-venv/bin/python -m py_compile python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_events.py python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_base_group_chat.py python/packages/autogen-agentchat/src/autogen_agentchat/teams/_group_chat/_base_group_chat_manager.py python/packages/autogen-agentchat/tests/test_group_chat.py
• git diff --check