Bug #119489 GTID tags can violate restrictions

Description:
According to the documentation, GTID tags cannot be longer than 8 characters. However, in reality, you can set the tag up to 42 characters long.

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mysql [localhost:8407] {msandbox} (world) > SET gtid_next='AUTOMATIC:thisislongerthan10_chars';
Query OK, 0 rows affected (0.00 sec)

mysql [localhost:8407] {msandbox} (world) > CREATE TABLE authors (id int primary key, name varchar(20));
Query OK, 0 rows affected (0.03 sec)

mysql [localhost:8407] {msandbox} (world) > INSERT INTO authors VALUES (1, 'Alice');
Query OK, 1 row affected (0.01 sec)

mysql [localhost:8407] {msandbox} (world) > SHOW BINLOG EVENTS IN 'binlog.000002';
+---------------+-----+-----------------------+-----------+-------------+--------------------------------------------------------------------------------------------+
| Log_name      | Pos | Event_type            | Server_id | End_log_pos | Info                                                                                       |
+---------------+-----+-----------------------+-----------+-------------+--------------------------------------------------------------------------------------------+
| binlog.000002 |   4 | Format_desc           |         1 |         127 | Server ver: 8.4.7, Binlog ver: 4                                                           |
| binlog.000002 | 127 | Previous_gtids        |         1 |         158 |                                                                                            |
| binlog.000002 | 158 | Gtid_tagged_log_event |         1 |         253 | SET @@SESSION.GTID_NEXT= '00008407-0000-0000-0000-000000008407:thisislongerthan10_chars:1' |
| binlog.000002 | 253 | Query                 |         1 |         402 | use `world`; CREATE TABLE authors (id int primary key, name varchar(20)) /* xid=20 */      |
| binlog.000002 | 402 | Gtid_tagged_log_event |         1 |         497 | SET @@SESSION.GTID_NEXT= '00008407-0000-0000-0000-000000008407:thisislongerthan10_chars:2' |
| binlog.000002 | 497 | Query                 |         1 |         573 | BEGIN                                                                                      |
| binlog.000002 | 573 | Table_map             |         1 |         635 | table_id: 103 (world.authors)                                                              |
| binlog.000002 | 635 | Write_rows            |         1 |         681 | table_id: 103 flags: STMT_END_F                                                            |
| binlog.000002 | 681 | Xid                   |         1 |         712 | COMMIT /* xid=21 */                                                                        |
+---------------+-----+-----------------------+-----------+-------------+--------------------------------------------------------------------------------------------+
9 rows in set (0.00 sec)

mmysql [localhost:8407] {msandbox} (world) > SET gtid_next = 'AUTOMATIC:this_is_a_very_long_gtid_tag_1234';
ERROR 1774 (HY000): Malformed GTID specification 'AUTOMATIC:this_is_a_very_long_gtid_tag_1234'.
mysql [localhost:8407] {msandbox} (world) > SET gtid_next = 'AUTOMATIC:this_is_a_very_long_gtid_tag_123';
Query OK, 0 rows affected (0.00 sec)

mysql [localhost:8407] {msandbox} (world) > SELECT LENGTH(@@gtid_next);
+---------------------+
| LENGTH(@@gtid_next) |
+---------------------+
|                  42 |
+---------------------+
1 row in set (0.00 sec)

mysql [localhost:8407] {msandbox} (world) > SELECT @@gtid_next;
+--------------------------------------------+
| @@gtid_next                                |
+--------------------------------------------+
| AUTOMATIC:this_is_a_very_long_gtid_tag_123 |
+--------------------------------------------+
1 row in set (0.00 sec)

Ref: https://dev.mysql.com/doc/refman/8.4/en/replication-options-gtids.html#sysvar_gtid_next

How to repeat:
See above. Simply set the tag longer than 8 characters, in violation of the docs specification.

Suggested fix:
Enforce constraint as stated in docs, or update docs to match actual constraint.

Source code indicates tags can be up to 32 characters long.

https://github.com/mysql/mysql-server/blob/8.4/libs/mysql/gtid/gtid_constants.h#L43-L44

mysql [localhost:8407] {msandbox} (world) > SELECT @@gtid_next, LENGTH('this_is_a_very_long_gtid_tag_123');
+--------------------------------------------+--------------------------------------------+
| @@gtid_next                                | LENGTH('this_is_a_very_long_gtid_tag_123') |
+--------------------------------------------+--------------------------------------------+
| AUTOMATIC:this_is_a_very_long_gtid_tag_123 |                                         32 |
+--------------------------------------------+--------------------------------------------+
1 row in set (0.00 sec)

Correction on opening description: The tag itself is not 42 characters, it can be 32 + 'AUTOMATIC:'