brokerd: state.sock answers memoryd's question about a session's state

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/brokerd/tests/serve.rs

@@ -63,7 +63,9 @@ impl Home {
         command
     }
 
-    /// Starts `brokerd serve` and waits until both sockets answer.
+    /// Starts `brokerd serve` and waits until both sockets answer. A socket accepts connections
+    /// as soon as it is bound, before `brokerd` has printed its startup notices; the admin
+    /// socket's answer comes from a handler thread, which runs only after they are printed.
     fn serve(&self) -> Running {
         let child = self
             .command(&[])
@@ -79,6 +81,11 @@ impl Home {
             assert!(Instant::now() < until, "brokerd never listened");
             std::thread::sleep(Duration::from_millis(20));
         }
+        let answer = exchange(&self.admin(), 1, Message::Approvals(Empty {}));
+        assert!(
+            matches!(&answer[0].msg, Message::ApprovalList(_)),
+            "{answer:?}"
+        );
         running
     }