fix(bridge): reset expiry timer on first signature after burn/refund expiry

After on_finalize expires a burn or refund transaction, re-proposals add
signatures via add_stellar_sig_burn_transaction / add_stellar_sig_refund_transaction
but tx.block was never updated. This caused on_finalize to immediately
re-expire the transaction in the same block the new signature was added
(since current_block - old_block >= RetryInterval was still true).

Also removes dead code in propose_stellar_burn_transaction_or_add_sig where
a redundant contains_key check made the block-update branch unreachable.

Closes #1080

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sameh-farouk

substrate-node/pallets/pallet-tft-bridge/src/tft_bridge.rs

@@ -240,16 +240,9 @@ impl<T: Config> Pallet<T> {
             Error::<T>::BurnTransactionAlreadyExecuted
         );
 
-        let Some(mut burn_tx) = BurnTransactions::<T>::get(tx_id) else {
-            return Err(DispatchErrorWithPostInfo::from(
-                Error::<T>::BurnTransactionNotExists,
-            ));
-        };
-
-        ensure!(
-            BurnTransactions::<T>::contains_key(tx_id),
-            Error::<T>::BurnTransactionNotExists
-        );
+        let burn_tx = BurnTransactions::<T>::get(tx_id).ok_or(
+            DispatchErrorWithPostInfo::from(Error::<T>::BurnTransactionNotExists),
+        )?;
 
         ensure!(
             burn_tx.amount == amount,
@@ -260,26 +253,12 @@ impl<T: Config> Pallet<T> {
             Error::<T>::WrongParametersProvided
         );
 
-        if BurnTransactions::<T>::contains_key(tx_id) {
-            return Self::add_stellar_sig_burn_transaction(
-                tx_id,
-                signature,
-                stellar_pub_key,
-                sequence_number,
-            );
-        }
-
-        let now = <frame_system::Pallet<T>>::block_number();
-
-        burn_tx.block = now;
-        burn_tx.sequence_number = sequence_number;
-        BurnTransactions::<T>::insert(tx_id.clone(), &burn_tx);
-
-        Self::add_stellar_sig_burn_transaction(tx_id, signature, stellar_pub_key, sequence_number)?;
-
-        Self::deposit_event(Event::BurnTransactionProposed(tx_id, target, amount));
-
-        Ok(().into())
+        Self::add_stellar_sig_burn_transaction(
+            tx_id,
+            signature,
+            stellar_pub_key,
+            sequence_number,
+        )
     }
 
     pub fn add_stellar_sig_burn_transaction(
@@ -319,6 +298,14 @@ impl<T: Config> Pallet<T> {
             stellar_pub_key,
         };
 
+        // Reset the expiry timer when this is the first signature after an expiry.
+        // on_finalize clears signatures and sets block to the expiry block. Without
+        // this reset, the old block value causes on_finalize to immediately re-expire
+        // the burn in the same block the new signature is added.
+        if tx.signatures.is_empty() {
+            tx.block = <frame_system::Pallet<T>>::block_number();
+        }
+
         tx.sequence_number = sequence_number;
         tx.signatures.push(stellar_signature.clone());
         BurnTransactions::<T>::insert(tx_id, &tx);
@@ -397,6 +384,11 @@ impl<T: Config> Pallet<T> {
             stellar_pub_key,
         };
 
+        // Reset the expiry timer when this is the first signature after an expiry.
+        if tx.signatures.is_empty() {
+            tx.block = <frame_system::Pallet<T>>::block_number();
+        }
+
         tx.sequence_number = sequence_number;
         tx.signatures.push(stellar_signature.clone());
         RefundTransactions::<T>::insert(&tx_hash, &tx);