fix(rpc): eth_estimateGas rejects unfunded accounts with 'insufficient funds for l1 fee' when gasPrice is zero or omitted

[panos-xyz]

Description

In Ethereum and morph-geth, callers (wallets, dApps) routinely invoke eth_estimateGas without specifying gasPrice or maxFeePerGas, often from a new or unfunded account (balance = 0), to determine how much gas a transaction or contract interaction will consume.

In morph-geth, unmetered gas estimation (where feeCap == 0) skips account balance capping, allowing unfunded accounts to estimate gas. However, in morph-reth, calling eth_estimateGas for an unfunded account without specifying gasPrice unconditionally fails with "insufficient funds for l1 fee".

Root Cause

In crates/rpc/src/eth/call.rs:105-121:

let balance = db
    .basic(tx_env.caller())
    .map_err(|e| MorphEthApiError::Eth(e.into()))?
    .map(|acc| acc.balance)
    .unwrap_or_default();
let available = balance
    .checked_sub(tx_env.value())
    .ok_or(MorphEthApiError::InsufficientFundsForTransfer)?;

// Reject at `l1_fee >= available` so the RPC surfaces the real
// reason rather than a downstream "gas required exceeds allowance 0".
if l1_fee >= available {
    return Err(MorphEthApiError::InsufficientFundsForL1Fee);
}
Ok(gas_allowance_from_balance(
    available - l1_fee,
    tx_env.gas_price(),
))

if l1_fee >= available is checked unconditionally before gas_allowance_from_balance. If an account has balance = 0 (or balance < l1_fee), available is 0 while l1_fee is positive, causing the check to reject the call with InsufficientFundsForL1Fee.

In contrast, in morph-geth (internal/ethapi/api.go:1409):

if feeCap.BitLen() != 0 {
    ...
    if l1DataFee.Cmp(available) >= 0 {
        return 0, errors.New("insufficient funds for l1 fee")
    }
    allowance := new(big.Int).Div(available, feeCap)
    ...
}

morph-geth guards the entire balance and L1 fee check with if feeCap.BitLen() != 0. When no gasPrice / maxFeePerGas is provided, feeCap is 0, so geth skips the balance recap and proceeds with gas estimation.

Similarly, upstream reth's caller_gas_allowance returns u64::MAX immediately when tx_env.gas_price() == 0.

Expected Behavior

When tx_env.gas_price() == 0 (unpriced estimation), caller_gas_allowance should not reject on l1_fee >= available, matching morph-geth and upstream reth.