statistics: TestDropPartitionStats can hit cleanup timeout in CI due to unclosed internal RecordSet

## Bug Report

### 1. Minimal reproduce step (Required)

Observed on PR `#67814`:
- failing check: `idc-jenkins-ci-tidb/unit-test`
- failing Bazel target: `//pkg/executor/test/simpletest:simpletest_test (shard 8 of 12)`

The failing test is:
- `TestDropPartitionStats`

CI logs show the same pattern across retries:

```text
=== RUN TestDropPartitionStats
...
[db.go:625] ["Waiting for closer"]
--- PASS: TestDropPartitionStats (116.41s / 118.14s / 125.44s)
-- Test timed out --
```

So the test body passes, but the process stalls during cleanup and then hits Bazel timeout.

The suspicious path is:
- `TestDropPartitionStats`
- `flush stats_delta *.*`
- `DumpStatsDeltaToKV(true)`
- `pkg/statistics/handle/storage/update.go:UpdateStatsMeta`

`UpdateStatsMeta` executes two internal `SELECT ... FOR UPDATE` SQLs via `statsutil.ExecWithCtx(...)`, but the returned `RecordSet` is not explicitly closed.

I made a minimal local validation patch that only closes those two internal `RecordSet`s, then reran:

```bash
cd pkg/executor/test/simpletest
go test -race -run TestDropPartitionStats -tags=intest,deadlock -count=20 -timeout=20m
```

Result:
- before patch: `104.291s`
- after patch: `65.068s`

This strongly suggests the unclosed `RecordSet` is delaying cleanup and making CI timeout much more likely.

### 2. What did you expect to see? (Required)

- `TestDropPartitionStats` should finish and clean up normally in CI.
- Internal stats SQL should not leave resources that delay `domain close` / `store close`.

### 3. What did you see instead (Required)

- `TestDropPartitionStats` passes its assertions, but cleanup stalls at badger `Waiting for closer`.
- The same timeout pattern happens repeatedly in CI.
- Explicitly closing the internal `RecordSet` significantly reduces local runtime.

### 4. What is your TiDB version? (Required)

Observed on PR `#67814`:
- commit: `4965430ff5c6d779d798ca62d550948da5b44e65`

Related context:
- suspected root-cause PR: `#58791`
- test-path changes: `#65870`, `#67728`

Suggested labels:
- `type/bug`
- `type/regression`
- `component/statistics`

Start with TestDropPartitionStats in pkg/executor/test/simpletest and trace the flush stats_delta path into pkg/statistics/handle/storage/update.go:UpdateStatsMeta. Run the provided race-enabled test command and compare cleanup behavior; done means the test completes without a cleanup timeout and internal stats SQL resources no longer delay domain or store closure.

Written by the indexing model from the issue text.