[dv/alert_handler] Functional coverage 100%

### Description

Some functional coverage is not 100% - https://reports.opentitan.org/hw/top_earlgrey/ip_autogen/alert_handler/dv/latest/cov_report/groups.html

Might need to change the randomization or modify cover bins.

Start with the linked alert_handler functional coverage report and identify the uncovered groups or bins. Then inspect the alert_handler design-verification randomization and coverage-bin definitions to determine whether the gaps need stimulus changes or coverage updates; done means the functional coverage reaches 100%.

Written by the indexing model from the issue text.