Estimate gas at pending block to fix FailedToObtainBlockhash

[syntrust]

This PR is another solution as an alternative to #506.

Rationale

The EVM specification requires that BLOCKHASH can only access the most recent 256 generated blocks, i.e., [block.number - 256, block.number - 1], excluding the current block itself.

eth_estimateGas executes in the context of the latest chain head block N by default, so block.number = N.
If BLOCKHASH(N) is called, since N >= block.number, it returns 0.

When explicitly using the pending block tag, the execution context changes. In Geth, block.number = N + 1, i.e., the next block being constructed. Therefore, BLOCKHASH(N) becomes valid (since N = block.number - 1).

In real execution, the transaction is included in block N + k (where k ≥ 1), so block.number = N + k. In the common case where k = 1, this matches the pending context.

Tests

Tests have been performed on the local testnet using extra temporary debug messages to confirm that the mining tx is completed successfully in the case of mining and estimation occurring in the same slot.

[qzhodl]

Do we also need to remove the logic that waits for one slot before calling estimateGas?

[syntrust]

Do we also need to remove the logic that waits for one slot before calling estimateGas?

Done.