Constant VARIANT_KEY_HINT 

pub const VARIANT_KEY_HINT: &str = "VARIANT values only have byte-wise equality and ordering, so \
    they cannot be used as a key: in a primary key, index, grouping or join key, ORDER BY, or set \
    operation.";

Shared hint for every VARIANT-as-key rejection, so a test can assert one stable string regardless of which layer fired.