[kmac] Harden FIFO bypass control

The new message FIFO features a bypass mode so shared messages can be sent directly to the hashing backend.

The control signal for this bypass functionality poses an attack surface as identified in https://github.com/lowRISC/opentitan/pull/30099#discussion_r3246440966.

@vogelpi proposed a mitigation in the same discussion (https://github.com/lowRISC/opentitan/pull/30099#discussion_r3246859593).

Start with PR #30099 and the two linked discussion comments, then trace the KMAC message FIFO bypass control to the hashing backend. Implement the mitigation proposed by @vogelpi and verify that the bypass control no longer exposes the identified attack surface.

Written by the indexing model from the issue text.