Implement BulkAction processor and validator on the pure-ABC base

Build the execution framework (processor, runner, validator) for the new self-contained bulk action base, independent of the legacy BaseAction hierarchy.

Target base: src/ai/backend/manager/actions/bulk/base.py (BaseBulkAction, pure ABC). Contract: entity_type() -> EntityType, entity_ids() -> Sequence[str], required_permission() -> Permission.

- Add a BulkActionProcessor (and its runner) bound to BaseBulkAction, under actions/bulk/.
- Add a BulkActionValidator that iterates entity_ids() and aggregates per-entity denials (partial failures allowed).
- Do not reuse the legacy ActionRunner typed to BaseAction; the new path must depend only on the pure-ABC base.

JIRA Issue: BA-6456