When you switched away from an ACP thread, the `AcpThreadView` entity (and thus thread, and subprocess) was leaked. This happened because we were using `cx.processor` for the `list` state callback, which uses a strong reference. This PR changes the callback so that it holds a weak reference, and adds some tests and assertions at various levels to make sure we don't reintroduce the leak in the future. Release Notes: - N/A