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

[EvanYao826]

Why are these changes needed?

Implements the feature requested in #6085: expose the group chat message thread as a public async API on BaseGroupChat.

Changes

_events.py — new GroupChatGetThread / GroupChatThreadResponse RPC message types for the protocol.

_base_group_chat_manager.py — new @rpc handler handle_get_thread() that returns a shallow copy of the internal self._message_thread.

_base_group_chat.py — new async def get_thread() on BaseGroupChat:

• Tracks messages in self._thread during run_stream, cleared on reset()
• Returns a snapshot (shallow copy) so callers cannot mutate internal state
• Works both during a run and after completion — no dependency on runtime message processing (avoids the embedded-runtime-stops-after-run issue)

tests/test_group_chat_get_thread.py — 4 pytest-asyncio tests:

• 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 internal state
• test_get_thread_with_no_task — works after run with no initial task

Design note

Unlike pause() / resume() which use send_message during an active run, get_thread() must work after run() completes. Since the embedded SingleThreadedAgentRuntime stops processing messages after stop_when_idle(), a pure send_message-based approach would hang. Instead, BaseGroupChat maintains its own self._thread list populated during run_stream, which the manager also populates via the shared output message queue. This keeps the API reliable regardless of runtime lifecycle.

Related issue number

Closes #6085

Checks

• I have included tests
• All 4 tests pass locally (pytest-asyncio)
• No existing tests broken

[EvanYao826]

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