fix: use lower fair l2 gas price for cbt (#2690)

this PR reduces fair l2 gas price for chains that use custom base token.
This is needed because CBT quote might fluctuate a lot for volatile
tokens. By the time a transaction reaches the server, it could have
already changed. Such transactions might fail validation without even
getting into the mempool.

core/node/api_server/src/tx_sender/mod.rs

@@ -517,7 +517,12 @@ impl TxSender {
             );
             return Err(SubmitTxError::GasLimitIsTooBig);
         }
-        if tx.common_data.fee.max_fee_per_gas < fee_input.fair_l2_gas_price().into() {
+
+        // At the moment fair_l2_gas_price is rarely changed for ETH-based chains. But for CBT
+        // chains it gets changed every few blocks because of token price change. We want to avoid
+        // situations when transactions with low gas price gets into mempool and sit there for a
+        // long time, so we require max_fee_per_gas to be at least current_l2_fair_gas_price / 2
+        if tx.common_data.fee.max_fee_per_gas < (fee_input.fair_l2_gas_price() / 2).into() {
             tracing::info!(
                 "Submitted Tx is Unexecutable {:?} because of MaxFeePerGasTooLow {}",
                 tx.hash(),