With character_set_connection=utf16 a valid SET time_zone='+00:00' is rejected with ER_UNKNOWN_TIME_ZONE because the raw multi-byte sysvar value is passed to my_tz_find() without charset conversion

`Sys_var_tz::do_check()` validates the new `time_zone` value by calling
`val_str()` on the item and passing the returned bytes to `my_tz_find()`. When the
connection character set is a multi-byte non-ASCII set (utf16), the literal written in
the statement is converted to utf16 bytes, but the returned buffer is a plain
`latin1`/byte buffer that is not converted back to the system character set before
`my_tz_find()` performs its offset parsing and hash lookup. Valid values such as
`'+00:00'` therefore never match and the assignment fails with
`ER_UNKNOWN_TIME_ZONE` (`ERROR 1298`).

The interesting part is that the original finding's gate was wrong: `SET NAMES utf16` is
itself rejected (`ERROR 1231` for `character_set_client`), but
`SET character_set_connection=utf16` alone is accepted and is sufficient to reproduce the
`time_zone` failure.

1. 1. Steps to reproduce (product observation)

```sql
– control
SET time_zone='+00:00'; – OK, @@session.time_zone = '+00:00'

– the gate that does NOT work (documented for completeness)
SET NAMES utf16; – ERROR 1231: Variable 'character_set_client' can't be set to the value of 'utf16'

– actual reproduction
SET character_set_connection=utf16;
SET time_zone='+00:00'; – ERROR 1298 (HY000): Unknown or incorrect time zone: '+00:00'
```

Observed: the control succeeds, `SET NAMES utf16` fails as above, and with
`character_set_connection=utf16` a valid `time_zone` value is rejected. Machine
evidence: `mariadb_reproduction_results/dsh_mdsh03_20261023_run2/result.json`,
SHA-256 `9b8f1710f7bdb39e431cc427c21c6b65a3bb9f3d4ead2720aee96c03a7d35023`.

1. 1. Expected result

`SET time_zone='+00:00'` should be accepted regardless of
`character_set_connection`: the sysvar check should convert the value to the system
character set (or compare it at the byte level after a well-defined conversion) before
calling `my_tz_find()`. If a given connection character set is unsupported for this
sysvar, the error should say so instead of reporting an unknown time zone.