Ensure a non-UTF8 charset cannot be used for protocol exchanges

he driver encodes and decodes all character data, assuming the connection character set is UTF-8. The server can, however, announce a change of character_set_client mid-session through the OK-packet session-state-tracking mechanism (e.g. a SET NAMES … run by a stored routine/trigger, server configuration, or a hostile server).

If the new charset is not UTF-8, the driver keeps reading and writing UTF-8 while the server interprets the same bytes under a different encoding, causing silent data corruption and a client/server charset-confusion mismatch.

Goal is that once the connection is fully initialized, any subsequent charset change to a value that is not utf8 / utf8mb3 / utf8mb4 is rejected: the driver raises R2dbcNonTransientResourceException (SQLState 08000) and closes the connection rather than continuing to exchange data under a mismatched encoding.

Reported by fg0x0.