BatchScanRegions should check context

## Enhancement Task

Now, BatchScanRegions ignores ctx and holds RegionTree read lock, which will block heartbeats if it hold lock for long periods.

Under heavy batch-scan traffic, requests can run for 10+ minutes, keeping the read lock and starving writes (CheckAndPutRootTree uses r.t.Lock()). This blocks region heartbeat updates.

```
[2026/01/01 00:22:46.092 +08:00] [WARN] [interceptor.go:197] ["request stats"] ["start time"=2026/01/01 00:11:09.834 +08:00] ["time spent"=11m36.258304292s]
```

BatchScanRegions should stop scanning on cancellation/deadline and we should reduce log