get_custody_columns: Move node_id test against UINT256_MAX out of the while loop.

[nalepae]

node_id is not modified in the loop, and moving the check against UINT256_MAX does not impact specification readability.

[zilm13]

Just noticed this part of code. This change will not fix the issue, because increment grows with every iteration, we could hit overflow on any step. We need something like this (but I don't like name tmp_id):

def get_custody_columns(node_id: NodeID, custody_subnet_count: uint64) -> Sequence[ColumnIndex]:
    assert custody_subnet_count <= DATA_COLUMN_SIDECAR_SUBNET_COUNT

    subnet_ids = []
    tmp_id = uint256(node_id)
    while len(subnet_ids) < custody_subnet_count:
        if tmp_id == UINT256_MAX:
            tmp_id = 0

        subnet_id = (
            bytes_to_uint64(hash(uint_to_bytes(tmp_id))[0:8])
            % DATA_COLUMN_SIDECAR_SUBNET_COUNT
        )
        if subnet_id not in subnet_ids:
            subnet_ids.append(subnet_id)
        tmp_id += 1
    assert len(subnet_ids) == len(set(subnet_ids))

    columns_per_subnet = NUMBER_OF_COLUMNS // DATA_COLUMN_SIDECAR_SUBNET_COUNT
    return sorted([
        ColumnIndex(DATA_COLUMN_SIDECAR_SUBNET_COUNT * i + subnet_id)
        for i in range(columns_per_subnet)
        for subnet_id in subnet_ids
    ])

[hwwhww]

closing via #3748