fix(#937): skip scheduled delivery if target already has pending scheduled message

hive-c0re/src/broker.rs

@@ -263,6 +263,21 @@ impl Broker {
         Ok(u64::try_from(n.max(0)).unwrap_or(0))
     }
 
+    /// Returns true when the recipient already has at least one
+    /// undelivered message from `sender` in the broker. Used by the
+    /// scheduler to avoid piling up repeated prompts when an agent is
+    /// slow or was briefly offline.
+    pub fn has_pending_from(&self, recipient: &str, sender: &str) -> Result<bool> {
+        let conn = self.conn.lock().unwrap();
+        let n: i64 = conn.query_row(
+            "SELECT COUNT(*) FROM messages
+              WHERE recipient = ?1 AND sender = ?2 AND delivered_at IS NULL",
+            params![recipient, sender],
+            |row| row.get(0),
+        )?;
+        Ok(n > 0)
+    }
+
     /// Long-poll variant of `recv_batch`: returns immediately if any
     /// row is pending (popping up to `max`); otherwise waits up to
     /// `timeout` for the broker to emit a `Sent { to: recipient }`