Probes a string primary key column. Only supports utf8mb4_bin against CHAR/VARCHAR
columns up to 768 characters. Enforcement is deferred to the caller. There may be
other collations we can support, but we should do more validation.
The longest key the probe bounds cover, in characters.
https://dev.mysql.com/doc/refman/8.4/en/innodb-limits.html caps an index
key at 3072 bytes, or 768 utf8mb4 characters. Longer keys (possible
through prefix indexes or narrower charsets) are not supported.