fix: MySQL 9.x charset handling and log_last_insert_id test race

- validate_charset: compare server_version as a numeric major
  version instead of testing the first character, so MySQL 9.x
  and later are treated like 8.x for collations with id >= 255
- test_log_last_insert_id-t: re-enable PROXYSQL FLUSH LOGS; with
  buffered query logging, eventslog_flush_timeout=0 alone still
  races the periodic flush thread

lib/MySQL_Variables.cpp

@@ -337,7 +337,9 @@ bool validate_charset(MySQL_Session* session, int idx, int &_rc) {
 		unsigned int replace_collation_nr = 0;
 		std::stringstream ss;
 		int charset = atoi(mysql_variables.client_get_value(session, idx));
-		if (charset >= 255 && myconn->mysql->server_version[0] != '8') {
+		// Pre-8.0 MySQL cannot handle collations with id >= 255.
+		// MySQL 8.x, 9.x and later can — match by numeric major version, not first character.
+		if (charset >= 255 && atoi(myconn->mysql->server_version) < 8) {
 			switch(mysql_thread___handle_unknown_charset) {
 				case HANDLE_UNKNOWN_CHARSET__DISCONNECT_CLIENT:
 					snprintf(msg,sizeof(msg),"Can't initialize character set %s", mysql_variables.client_get_value(session, idx));

test/tap/tests/test_log_last_insert_id-t.cpp

@@ -77,11 +77,10 @@ int main(int argc, char** argv) {
 	MYSQL_QUERY(proxysql_mysql, "INSERT INTO test.test_insert_id VALUES (NULL)");
 	MYSQL_QUERY(proxysql_mysql, "DO 1");
 
-	/* in 3.0.6 we added buffering for query log . So we either flush logs or set mysql-eventslog_flush_timeout = 0 */
-	/*
+	/* in 3.0.6 we added buffering for query log; flush_timeout=0 alone leaves a race
+	 * between query completion and the periodic flush thread, so force a flush here. */
 	MYSQL_QUERY(proxysql_admin, "PROXYSQL FLUSH LOGS");
 	sleep(1);
-	*/
 	{
 		const string f_path { get_env("REGULAR_INFRA_DATADIR") + "/loginsertid.log.00000001" };
 		diag("Trying to open file %s" , f_path.c_str());