fix: Adjust invariant checks when selecting participants

[kuksag]

Context

The function:

pub fn select_random_active_participants_including_me(
        &self,
        total: usize,
        peers_to_consider: &[ParticipantId],
    ) -> anyhow::Result<Vec<ParticipantId>>

Should return total number of online participants amongst peers_to_consider, including the caller participant.

This function has two usages:

1. In generation of triples
2. In generation eddsa signatures

Bug

This function behaves incorrectly, when the size of peers_to_consider is insufficient, to make up for total participants.
Extrapolated example: if we pass empty peers_to_consider. In that case [me] will be returned, because me is getting appended regardless. The error should be returned instead.

IteratorRandom::choose_multiple(_, amount) silently returns less than amount elements, if there are not enough.

Solution

Make invariant check on return value

Effect

This has no practical effect at the moment, because all_participants_from_config is being passed into select_random_active_participants_including_me

[ghost]

That's a very good catch. Thanks for fixing it.

Could you also add a unit test, so we have some regression test for this bug? It's not a hard requirement from my side if it's tedious to test.

[netrome]

Some suggestions, otherwise lgtm.

[netrome]

I assume this failed due to main being broken for a brief time. Taking the liberty to add it to the merge queue again.