Hot read leader scheduling skips topology isolation when Placement Rules are disabled #11352

Bug Report

What did you do?

Configure location-labels and disable Placement Rules. Consider a hot region with replicas on Store A (z1, leader), Store C (z2), and Store D (z3). Store B is a healthy, lower-load Store in z2, but does not currently contain a peer of this region. Run the hot-region read scheduler.

What did you expect to see?

The leader move should preserve the existing replica topology isolation.

What did you see instead?

The scheduler can select Store B and create a MoveLeaderOperator. The resulting replicas would be on z2, z2, and z3, reducing the region's topology isolation.

In pkg/schedule/filter/filters.go at v8.5.6 ([lines 774–782](https://github.com/tikv/pd/blob/v8.5.6/pkg/schedule/filter/filters.go#L774-L782)), NewPlacementLeaderSafeguard returns nil when Placement Rules are disabled.

In pkg/schedule/schedulers/hot_region.go ([lines 987–1007](https://github.com/tikv/pd/blob/v8.5.6/pkg/schedule/schedulers/hot_region.go#L987-L1007)), the read hot-region path admits non-peer targets using only StoreStateFilter.

What version of PD are you using (pd-server -V)?

v8.5.6