fix(pxe): restore fee-payer public data write DA charge in kernelless gas metering

Block #24580's mechanical re-graft (99cbb5d) pulled
contract_function_simulator.ts from the v5 tip, which predates next's
5ca883c (add fixed gas cost for updating fee payer balance to
private-only tx). The Noir private-kernel gas meter (gas_meter.nr) charges
PUBLIC_DATA_WRITE_LENGTH DA fields for the fee-payer balance write on
private-only txs, so kernelless estimates under-counted DA gas by 64 on
every private-only tx (kernelless_simulation 128 vs 192 squashing case,
768 vs 832 account deployments; same drift behind the bot/amm/sponsored
"gas used exceeds the gas limits" failures for private-only txs).

Restore the isPrivateOnlyTx block in meterGasUsed to match next and the
kernels. Complements d526c2d (protocol nullifier injection), which fixed
the other half of the kernelless DA drift.

Verified locally: kernelless_simulation 7/7 (was 4/7),
private_payments.parallel 8/8, txe unit tests 17/17.

Author: Thunkar

yarn-project/pxe/src/contract_function_simulator/contract_function_simulator.ts

@@ -20,6 +20,7 @@ import {
   MAX_PRIVATE_LOGS_PER_TX,
   MAX_TX_LIFETIME,
   PRIVATE_TX_L2_GAS_OVERHEAD,
+  PUBLIC_DATA_WRITE_LENGTH,
   PUBLIC_TX_L2_GAS_OVERHEAD,
   TX_DA_GAS_OVERHEAD,
 } from '@aztec/constants';
@@ -924,6 +925,11 @@ function meterGasUsed(data: PrivateToRollupAccumulatedData | PrivateToPublicAccu
   );
   meteredL2Gas += numContractClassLogs * L2_GAS_PER_CONTRACT_CLASS_LOG;
 
+  if (isPrivateOnlyTx) {
+    // The public data write that the private tx base rollup always performs to update the fee payer's balance.
+    meteredDAFields += PUBLIC_DATA_WRITE_LENGTH;
+  }
+
   const meteredDAGas = meteredDAFields * DA_GAS_PER_FIELD;
 
   if ((data as PrivateToPublicAccumulatedData).publicCallRequests) {