Fix ACP connection and thread leak (#35670)

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

crates/agent_servers/src/acp/v0.rs

@@ -380,6 +380,7 @@ impl AcpConnection {
 
             let stdin = child.stdin.take().unwrap();
             let stdout = child.stdout.take().unwrap();
+            log::trace!("Spawned (pid: {})", child.id());
 
             let foreground_executor = cx.foreground_executor().clone();