fix: incorrect passing of Thread instance to Store.save_thread

[quettabit]

as Thread is passed to save_thread and given it already has items field, it leads to the following runtime error on the read path:

TypeError: chatkit.types.Thread() got multiple values for keyword argument 'items'

and it happens at:

chatkit-python/chatkit/server.py

Line 692 in 4656c56

return Thread(**thread_meta.model_dump(), items=thread_items)