Fix(evm): emit gas in ETHTransaction.toEstimate() when > 0 only

[KabaDH]

What

toEstimate() now adds a gas field to the eth_estimateGas payload when
gasLimit > 0. When gasLimit == 0 (auto-fill) the behavior is unchanged.

if (gasLimit > BigInt.zero) 'gas': '0x${gasLimit.toRadixString(16)}',

Why

Optimism: without gas, the node uses the block gas limit (~40M) as the upper bound.
Strict op-geth nodes run an affordability pre-check on that bound
(40M × gas_price > balance) and reject the request with intrinsic gas too high — breaking gas estimation for token transfers (legacy and EIP-1559). With
gas set, the bound becomes affordable and estimation succeeds.

Verified against live OP Sepolia nodes (identical payload):

Node	without gas	with gas
sepolia.optimism.io	❌ intrinsic gas too high	✅ 0xb173
optimism-sepolia-rpc.publicnode.com	❌ intrinsic gas too high	✅ 0xb173

Important

The error currently shows up on testnet (OP Sepolia): its op-geth already
ships the stricter gas estimator. On mainnet the node still silently caps the
bound, so there is no error yet. Once the network upgrade reaches mainnet, the
same behavior is expected to appear there too — so we land this fix ahead of
time, as insurance.

On every other geth network the change is safe: gas is added only when
gasLimit is explicitly set.

Tests

Tests added in test/etherum/transaction_test.dart.

[KabaDH]

Hi @mrtnetwork!
Could you please take a look at one more pull request when you have a chance? I'd really appreciate it. Thanks in advance!