Render error state when agent binary exits unexpectedly (#35651)

This PR adds handling for the case where an agent binary exits
unexpectedly after successfully establishing a connection.

Release Notes:

- N/A

---------

Co-authored-by: Agus <agus@zed.dev>

crates/agent_ui/src/agent_diff.rs

@@ -1523,7 +1523,8 @@ impl AgentDiff {
             }
             AcpThreadEvent::Stopped
             | AcpThreadEvent::ToolAuthorizationRequired
-            | AcpThreadEvent::Error => {}
+            | AcpThreadEvent::Error
+            | AcpThreadEvent::ServerExited(_) => {}
         }
     }