fix(eth): retry gas estimate without fee fields on base-fee rejection (Gnosis/Nethermind)

[CharlVS]

What

Make eth_estimateGas resilient to nodes that reject EIP-1559 fee fields when max_fee_per_gas is below the current block base fee. On such a rejection, the estimate now transparently retries without the fee fields (gas usage is independent of fee values).

Why

Nethermind v1.38.0 (released 2026-06-02, the client behind Gnosis) started validating the EIP-1559 fee fields during eth_estimateGas. When a non-zero fee field is present, ShouldValidateGas re-enables the base-fee/premium check even though the call runs with SkipValidation, so the call is rejected with "miner premium is negative" (renamed by PR #11190 to the geth-style "max fee per gas less than block base fee") whenever max_fee_per_gas < baseFee.

KDF intentionally passes fee fields into the estimate request for contracts that branch on gas price (e.g. TUSD — atomicDEX-API#643). When Gnosis's base fee spiked above our (high-policy) estimate, the ERC20 approve gas estimate was rejected, which broke the fee preimage and therefore order posting.

This is a behavior change on the node side (Nethermind's own PR #11192 is literally titled "removes MaxFeePerGas for eth_estimateGas"); the fix belongs on our end — a gas-usage estimate should not be gated by fee caps.

How

• is_max_fee_per_gas_below_base_fee_error() — single classifier recognizing all three node wordings: geth's "max fee per gas less than block base fee", older Nethermind's "fee cap less than block base fee", and the new "miner premium is negative".
• EthCoin::estimate_gas_with_pay_for_gas_option() — estimates with fee fields (preserving the TUSD behavior) and, on a base-fee rejection, retries without any fee fields. Both swap-side estimate paths (estimate_gas_for_contract_call, get_fee_to_send_taker_fee) route through it.
• Withdraw path (get_eth_gas_details_from_withdraw_fee) reuses the classifier so the new wording surfaces as the structured GasFeeCapBelowBaseFee/GasFeeCapTooLow error instead of a raw Transport error. It deliberately does not retry — withdraw uses a user-supplied fee that must be reported as too low.
• parse_fee_cap_error() extended to also parse Nethermind v1.38.0's maxFeePerGas: N, baseFee: N wording.

Reviewer notes

• The fee-less retry is safe on both clients: omitting all fee fields (incl. gas_price, which maps to both EIP-1559 fields for legacy txs in Nethermind) makes Nethermind's ShouldValidateGas return false and geth run with NoBaseFee, so the estimate succeeds regardless of base fee.
• The existing get_swap_fee_policy_for_estimate (always-High) mitigation is retained; the retry is the safety net for when even the High estimate's cap is exceeded.

Tests

• test_estimate_gas_retries_without_fee_fields_when_max_fee_below_base_fee — mocks a Nethermind-style rejection while fee fields are present, success once omitted; asserts exactly one fee-bearing attempt + one fee-less retry succeeds.
• test_estimate_gas_does_not_retry_on_unrelated_error — an unrelated error (execution reverted) propagates with no retry.
• test_parse_fee_cap_error — both geth and Nethermind wordings parse; the numberless message yields None.

cargo check -p coins --lib --tests, cargo fmt --check, and cargo clippy (zero new warnings) all pass; new + existing eth gas-estimate tests pass.

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