[MCS] scheduler show does not display handler-only schedulers (evict-leader-scheduler, etc.)

## Bug Report
After PR #7082 (introduced schedulerHandlers) In MCS mode, handler-only schedulers (e.g., `evict-leader-scheduler`) in API Service are stored in the `schedulerHandlers` map [`GetSchedulerNames()`](https://github.com/tikv/pd/blob/release-8.5-20251204-v8.5.4/pkg/schedule/schedulers/scheduler_controller.go#L92) only iterates the `schedulers` map and does not include the `schedulerHandlers` map. This causes `scheduler show` to fail to display these schedulers

### What did you do?
In MCS (Microservice) mode, after adding a scheduler via `pd-ctl scheduler add evict-leader-scheduler `, executing `scheduler show` does not display the scheduler.

### What did you expect to see?
`scheduler show` should display all added schedulers, including handler-only schedulers (e.g., `evict-leader-scheduler`).

### What did you see instead?
`scheduler show` does not display handler-only schedulers, even though the scheduler has been successfully added and is working correctly.

### What version of PD are you using (`pd-server -V`)?
v8.5.4