Conceal `Langchain::Thread` away when creating or interacting with `Langchain::Assistant`.

[Andrei Bondarev]

Description

When user is instantiating Langchain::Assistant, they're explicitly passing thread: Langchain::Thread.new in. This detail can be concealed away, it's an implementation detail.

Tasks

• Make thread: nil optional in the initializer.
• Create a new thread if no thread was passed in:

@thread = thread || Langchain::Thread.new