Investigate validate check for command defined but not used in event section
MR: Pending
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## Description
We recently spot [one bug](https://gitlab.com/gitlab-org/gitlab/-/issues/557586) for event using command, but the command is not defined.
Then following up on the discussion [here](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/199388#note_2654992068), we would like to check the reverse case, where command is defined, but no event is using the command.
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## Acceptance criteria
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