feat(auth): enhance plugin scheduler with HasScheduler support and fast-path tests

- Added `pluginSchedulerState` interface with `HasScheduler` method for improved plugin scheduler state checks.
- Updated `Manager.hasPluginScheduler` to handle `HasScheduler` logic.
- Implemented and tested fast-path handling for inactive plugin schedulers, including mixed provider scenarios.
- Expanded unit test coverage to ensure correct behavior in various scheduler states.

internal/pluginhost/scheduler.go

@@ -30,6 +30,10 @@ func (h *Host) PickAuth(ctx context.Context, req pluginapi.SchedulerPickRequest)
 	return resp, true, nil
 }
 
+func (h *Host) HasScheduler() bool {
+	return h.schedulerRecord() != nil
+}
+
 func (h *Host) schedulerRecord() *capabilityRecord {
 	if h == nil {
 		return nil