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 in composer.py:419-422 and inspect the existing validator in actions/v2/relation/validator/rbac.py. Wire the RBAC validator into the empty relation-action slot, then add a composition test confirming the validator is present so the slot cannot be left empty again.

Written by the indexing model from the issue text.