agent_ui: Add more improvements to the subagent UI (#50260)

This PR includes the following adjustments:

- Add divider in the full screen view to expose what part of a
subagent's output is actually sent to the parent model
- Auto-expand the subagent card if there's only one running on a turn
- Adjust errors display within the subagent card

Release Notes:

- N/A

crates/agent_ui/src/agent_diff.rs

@@ -1403,7 +1403,7 @@ impl AgentDiff {
                     self.update_reviewing_editors(workspace, window, cx);
                 }
             }
-            AcpThreadEvent::Stopped => {
+            AcpThreadEvent::Stopped(_) => {
                 self.update_reviewing_editors(workspace, window, cx);
             }
             AcpThreadEvent::Error | AcpThreadEvent::LoadError(_) | AcpThreadEvent::Refusal => {