Improve transaction status error handling

Introduce a dedicated error variable for transaction not found cases in the
adapter layer. This separates internal error handling from API errors and
follows Go best practices by using errors.Is() for comparisons.

The change:
- Adds errTransactionNotFound for internal error handling
- Replaces string-based error comparison with errors.Is()
- Maintains existing API behavior while improving code maintainability

rpc/transaction.go

@@ -565,6 +565,8 @@ func (h *Handler) AddTransaction(ctx context.Context, tx BroadcastedTransaction)
 	}, nil
 }
 
+var errTransactionNotFound = fmt.Errorf("transaction not found")
+
 func (h *Handler) TransactionStatus(ctx context.Context, hash felt.Felt) (*TransactionStatus, *jsonrpc.Error) {
 	receipt, txErr := h.TransactionReceiptByHash(hash)
 	switch txErr {
@@ -585,7 +587,7 @@ func (h *Handler) TransactionStatus(ctx context.Context, hash felt.Felt) (*Trans
 
 		status, err := adaptTransactionStatus(txStatus)
 		if err != nil {
-			if !errors.Is(err, fmt.Errorf("%s", ErrTxnHashNotFound.Message)) {
+			if !errors.Is(err, errTransactionNotFound) {
 				h.log.Errorw("Failed to adapt transaction status", "err", err)
 			}
 			return nil, ErrTxnHashNotFound
@@ -753,7 +755,7 @@ func adaptTransactionStatus(txStatus *starknet.TransactionStatus) (*TransactionS
 	case starknet.Received:
 		status.Finality = TxnStatusReceived
 	case starknet.NotReceived:
-		return nil, fmt.Errorf("%s", ErrTxnHashNotFound.Message)
+		return nil, errTransactionNotFound
 	default:
 		return nil, fmt.Errorf("unknown finality status: %v", finalityStatus)
 	}