Chore: rename bulk action/validator/processor to batch

Goal

Rename the bulk action infrastructure to "batch" for naming consistency with the original BA-6100 intent and to better reflect that items can mix RBAC element types.

Scope

- `BaseBulkAction` / `BaseBulkActionResult` → `BaseBatchAction` / `BaseBatchActionResult`

- `BulkActionValidator` / `BulkValidationResult` / `DeniedEntity` → batch counterparts

- `BulkActionProcessor` / `BulkProcessResult` / `ValidatorDecision` → batch counterparts

- `BulkActionRBACValidator` → `BatchActionRBACValidator`

- File paths: `actions/action/bulk.py` → `batch.py`, same for validator/processor/validators/rbac

- Tests: `test_bulk_processor.py` → `test_batch_processor.py`, and bulk sections in `test_rbac_validators.py`

Out of Scope

- Behavior changes — pure rename + restructure.

- Any new functionality on top of BA-6100.

Acceptance Criteria

- No public symbol containing "Bulk" remains in `actions/` and its tests.

- `pants fmt fix lint check test` all pass on touched paths.

- PR description notes that this is a follow-up to BA-6100.

Notes

BA-6100 introduces the underlying infrastructure (mixed element types + `fail_on_any_denial` policy) under the existing "bulk" name. This issue completes the naming alignment as a separate, low-risk chore.

JIRA Issue: BA-6112