[executor] build_tx missing max_fee >= max_priority_fee guard — invalid signed envelope reaches broadcast

[obchain]

PR: #41 (feat/executor: transaction builder + eth_call simulator)
File: crates/charon-executor/src/builder.rs — build_tx()
Refs #41

Problem

EIP-1559 requires max_fee_per_gas >= max_priority_fee_per_gas. build_tx accepts both as u128 with no validation:

pub async fn build_tx<P>(
    &self,
    provider: &P,
    calldata: Bytes,
    max_fee_per_gas: u128,
    max_priority_fee_per_gas: u128,
    gas_limit: u64,
) -> Result<TransactionRequest>

If a caller passes inverted values, alloy encodes the TransactionRequest successfully and sign() produces a valid signed envelope. The envelope is rejected by every BSC node at submission with "max priority fee per gas higher than max fee per gas". The error surfaces only at broadcast, not at build or sign time, making it a silent failure from the executor's perspective.

Fix

Add a guard at the start of build_tx:

if max_priority_fee_per_gas > max_fee_per_gas {
    anyhow::bail!(
        "max_priority_fee_per_gas ({max_priority_fee_per_gas}) > max_fee_per_gas ({max_fee_per_gas})"
    );
}

[obchain]

Resolved in ad042ec on feat/16-executor-builder-and-sim.