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, excessively long Client ID, thereby impacting the resulting shortened ID.

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