fix: make address prio queue inner private

To protect invariant.

crates/mempool/src/priority_queue.rs

@@ -61,13 +61,23 @@ impl PartialOrd for PrioritizedTransaction {
 
 // TODO: remove when is used.
 #[allow(dead_code)]
-// Assumption: there are no gaps, and the transactions are received in order.
-pub struct AddressPriorityQueue(pub Vec<ThinTransaction>);
+// Invariant: Transactions have strictly increasing nonces, without gaps.
+// Assumption: Transactions are provided in the correct order.
+#[derive(Default)]
+pub struct AddressPriorityQueue(Vec<ThinTransaction>);
 
 // TODO: remove when is used.
 #[allow(dead_code)]
 impl AddressPriorityQueue {
     pub fn push(&mut self, tx: ThinTransaction) {
+        if let Some(last_tx) = self.0.last() {
+            assert_eq!(
+                tx.nonce,
+                last_tx.nonce.try_increment().expect("Nonce overflow."),
+                "Nonces must be strictly increasing without gaps."
+            );
+        }
+
         self.0.push(tx);
     }