Respcet `clientid_prefix` config for MQTT bridges.

As of version 5.4.1, EMQX limits MQTT client ID lengths to 23 bytes.
Previously, the system included the `clientid_prefix` in the hash calculation of the original unique, but long client ID, thereby impacting the resulting shortened ID.

Change Details:
- Without Prefix: Behavior remains unchanged; EMQX will hash the long (> 23 bytes) client ID into a 23-byte space.
- With Prefix:
  - Prefix no more than 19 bytes: The prefix is preserved, and the client ID is hashed into a 4-byte space capping the length within 23 bytes.
  - Prefix is 20 or more bytes: EMQX no longer attempts to shorten the client ID, respecting the configured prefix in its entirety.