feat(agentchat): add get_thread() to BaseGroupChat (closes #6085)

[seetaram-codebase]

Why are these changes needed?

Implements the feature requested in #6085: expose the group chat manager's internal _message_thread as a public async API on BaseGroupChat.

Changes

_events.py — new GroupChatGetThreadEvent RPC message type (empty model, like GroupChatReset).

_base_group_chat_manager.py — new @rpc handler handle_get_thread() that returns list(self._message_thread) (shallow copy to protect internal state).

_base_group_chat.py — new async def get_thread() -> List[BaseAgentEvent | BaseChatMessage] on BaseGroupChat that sends the RPC to the manager and returns the result. Works during a run and after completion; handles both embedded and external runtimes.

tests/test_group_chat_get_thread.py — 4 pytest-asyncio tests (embedded + external runtime fixtures):

• test_get_thread_after_run — non-empty snapshot matches TaskResult messages
• • test_get_thread_not_initialized_raises — RuntimeError before first run
• • test_get_thread_returns_snapshot — mutating returned list does not affect manager
• • test_get_thread_while_running — concurrent calls during a live run return a growing thread

Related issue number

Closes #6085

Checks

• I've included tests corresponding to the change (4 new async tests)
• - [ ] All auto checks have passed (CI pending — get_thread() + get_thread_while_running may need runtime tuning)

[seetaram-codebase]

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