Migrate the idle checker assignment domain to the v2 processor registry

Move the assignment actions (admin create, admin_search, scoped_search, update, purge) from the legacy v1 processors (ActionProcessor / SingleEntityActionProcessor / BulkActionProcessor) to the v2 ProcessorRegistry, keeping the existing (UUID-based) API contracts unchanged - framework swap only. Delete services/idle_checker_assignment/service.py in favor of generic services and specs; update adapter call sites. The enable/disable re-contract happens in a later issue on top of this.

## Success Criteria
- [ ] all assignment actions run via the v2 registry with unchanged contracts
- [ ] services/idle_checker_assignment/service.py removed
- [ ] existing component permission tests still pass
- [ ] pants test passes for affected packages

JIRA Issue: BA-7277

Start with services/idle_checker_assignment/service.py and trace the assignment actions through ActionProcessor, SingleEntityActionProcessor, and BulkActionProcessor. Locate the v2 ProcessorRegistry and the adapter call sites, then run the existing component permission tests and pants test for affected packages. Done means all listed actions use the registry, contracts remain unchanged, and the legacy service is removed.

Written by the indexing model from the issue text.