Wire the RBAC validators into relation actions

The relation validator exists but the composer leaves its slot empty, so relation actions run without any permission check (composer.py:419-422, actions/v2/relation/validator/rbac.py). Wire it and pin the wiring with a composition test so an empty slot cannot recur.

JIRA Issue: BA-7556

Start at composer.py:419-422 and compare the empty validator slot with actions/v2/relation/validator/rbac.py. Wire the existing RBAC validator into the relation action composition, then add a composition test that fails if the slot is empty. Done means relation actions include the permission validator and the test protects that wiring.

Written by the indexing model from the issue text.