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/claude.rs

@@ -125,8 +125,7 @@ impl AgentConnection for ClaudeAgentConnection {
                         session_id.clone(),
                         &mcp_config_path,
                         &cwd,
-                    )
-                    .await?;
+                    )?;
 
                     let pid = child.id();
                     log::trace!("Spawned (pid: {})", pid);
@@ -262,7 +261,7 @@ enum ClaudeSessionMode {
     Resume,
 }
 
-async fn spawn_claude(
+fn spawn_claude(
     command: &AgentServerCommand,
     mode: ClaudeSessionMode,
     session_id: acp::SessionId,