test: Overhaul tests for new lease-based design

Updates all tests in the registry package to align with the new
lease-based API and simplified component responsibilities.

- `managedqueue_test` is rewritten to focus on stats propagation and
   invariants, removing all signal testing.
- `shard_test` is updated to reflect its simpler role and new draining
   logic.
- `registry_test` is completely overhauled to test the `WithConnection`
   API, JIT registration, and the new lease-based GC logic, using a fake
  clock for deterministic lifecycle testing.

Author: LukeAVanDrie