feat(op-reth/txpool): reserve the Isthmus operator fee in pool balance check

[asdv23]

Problem

Closes #21608.

OpTransactionValidator::apply_op_checks (rust/op-reth/crates/txpool/src/validator.rs) reserves only the L1 data fee (l1_tx_data_fee) when checking that a sender can afford a transaction, not the Isthmus operator fee. On a post-Isthmus OP chain a sender whose balance covers L2 gas + value + L1 data fee — but not the operator fee — is admitted to the pool and then fails at execution.

operator_fee did not appear anywhere in rust/op-reth/crates/txpool before this change.

op-geth already charges the operator fee in its txpool balance check (NewOperatorCostFunc / NewTotalRollupCostFunc, core/types/rollup_cost.go, combined via TotalTxCost and used in ValidateTransactionWithState), so op-reth was behind op-geth here.

Change

• Add operator_fee_addition() and fold its result into cost_addition in apply_op_checks.
• Guarded on Isthmus activation (via the spec resolved from chain spec + timestamp with revm_spec_by_timestamp_after_bedrock) and the presence of both operator fee params. L1BlockInfo::operator_fee_charge .expect()s the scalar and constant, so it panics when they are unset (pre-Isthmus); the guard returns zero without calling it.
• Uses the tx gas_limit (worst-case spend), matching op-geth's tx.Gas().
• cost() is left untouched — the operator fee stays an overlay applied at the check site, exactly like the existing L1 data fee.

Scope / safety

Pool-admission only — no block-validation / state-root / execution change, so no consensus impact. Sequencer-only concern (verifiers do not admit from the mempool). Returns to op-geth parity.

Not in scope

The cumulative pending-balance path (PoolInternalTransaction::next_cumulative_cost sums the intrinsic cost(), which omits both the L1 fee and the operator fee) is a separate concern and is not addressed here.

Sibling of the L1-info gas-limit reservation: #21546 / #21548.

Tests

Unit tests for operator_fee_addition:

• charge applied (equals operator_fee_charge) when Isthmus active + params present;
• zero before Isthmus;
• zero and no panic when params unset (pre-Isthmus);
• zero for deposit-typed (0x7E) input.

cargo test -p reth-optimism-txpool (operator_fee tests) and cargo clippy -p reth-optimism-txpool --all-targets pass; cargo +nightly fmt --check clean.