Return a propoer error code for txpool is full error

[angel-ding-cb]

Currently, when reth's transaction pool is full, it returns an RPC error without a specific error code. This leads to two issues:

1. Users receive ambiguous errors with code = None in the response
2. Monitoring systems (like proxyd) cannot properly categorize and alert on pool overflow conditions since the error lacks a standard error code

Solution
Map the RpcPoolError::TxPoolOverflow error to a proper JSON-RPC error code by modifying the error conversion implementation:

impl From<RpcPoolError> for jsonrpsee_types::error::ErrorObject<'static> {
    fn from(error: RpcPoolError) -> Self {
        match error {
            RpcPoolError::Invalid(err) => err.into(),
            RpcPoolError::TxPoolOverflow => rpc_error_with_code(
                EthRpcErrorCode::TransactionRejected.code(),
                error.to_string()
            ),
            error => internal_rpc_err(error.to_string()),
        }
    }
}

This change:

1. Maps pool overflow errors to the standard TransactionRejected error code (-32003)
2. Maintains the descriptive error message "txpool is full"
3. Keeps other error handling behavior unchanged

Benefits

1. Users receive clear error responses with proper error codes
2. Monitoring systems can properly track and alert on pool overflow conditions
3. Better alignment with Ethereum JSON-RPC error code standards

[mattsse]

cool, ty.

that makes sense