`CHECK INDEX` statement meets runtime error

## Bug Report

`CHECK INDEX` statement meets runtime error. It can be reproduced on the docker image `pingcap/tidb:nightly`.

### 1. Minimal reproduce step (Required)

```sql
CREATE TABLE t (c1 INT PRIMARY KEY, c2 JSON, INDEX i ((CAST(c2->'$.array' AS UNSIGNED ARRAY))));
INSERT INTO t (c1, c2) VALUES (1, '{"name": "John", "age": 30}');
ADMIN CHECK INDEX t i (500, 600);
```

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

No runtime error.

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

```sql
mysql> ADMIN CHECK INDEX t i (500, 600);
ERROR 1105 (HY000): runtime error: index out of range [3] with length 3
```

Logs:
```
[2025/07/16 14:12:00.800 +00:00] [INFO] [conn.go:1190] ["command dispatched failed"] [conn=2097160] [session_alias=] [connInfo="id:2097160, addr:172.17.0.1:36354 status:10, collation:utf8mb4_0900_ai_ci, user:root"] [command=Query] [status="inTxn:0, autocommit:1"]
[sql="ADMIN CHECK INDEX t i (500, 600)"] [txn_mode=PESSIMISTIC] [timestamp=459453266866733056]
runtime error: index out of range [3] with length 3
github.com/pingcap/errors.Trace
/root/go/pkg/mod/github.com/pingcap/errors@v0.11.5-0.20250523034308-74f78ae071ee/juju_adaptor.go:15
github.com/pingcap/tidb/pkg/util.GetRecoverError
/workspace/source/tidb/pkg/util/util.go:312
github.com/pingcap/tidb/pkg/executor/internal/exec.Next.func1
/workspace/source/tidb/pkg/executor/internal/exec/executor.go:444
runtime.gopanic
/root/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.11.linux-amd64/src/runtime/panic.go:791
runtime.goPanicIndex
/root/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.11.linux-amd64/src/runtime/panic.go:115
encoding/binary.littleEndian.Uint32
/root/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.11.linux-amd64/src/encoding/binary/binary.go:87
github.com/pingcap/tidb/pkg/util/chunk.(*Codec).decodeColumn
/workspace/source/tidb/pkg/util/chunk/codec.go:111
github.com/pingcap/tidb/pkg/util/chunk.(*Codec).DecodeToChunk
/workspace/source/tidb/pkg/util/chunk/codec.go:98
github.com/pingcap/tidb/pkg/util/chunk.(*Decoder).Reset
/workspace/source/tidb/pkg/util/chunk/codec.go:262
github.com/pingcap/tidb/pkg/distsql.(*selectResult).readFromChunk
/workspace/source/tidb/pkg/distsql/select_result.go:493
github.com/pingcap/tidb/pkg/distsql.(*selectResult).Next
/workspace/source/tidb/pkg/distsql/select_result.go:436
github.com/pingcap/tidb/pkg/executor.(*CheckIndexRangeExec).Next
/workspace/source/tidb/pkg/executor/admin.go:72
github.com/pingcap/tidb/pkg/executor/internal/exec.Next
/workspace/source/tidb/pkg/executor/internal/exec/executor.go:460
github.com/pingcap/tidb/pkg/executor.(*ExecStmt).next
/workspace/source/tidb/pkg/executor/adapter.go:1322
github.com/pingcap/tidb/pkg/executor.(*recordSet).Next
/workspace/source/tidb/pkg/executor/adapter.go:173
github.com/pingcap/tidb/pkg/server/internal/resultset.(*tidbResultSet).Next
/workspace/source/tidb/pkg/server/internal/resultset/resultset.go:72
github.com/pingcap/tidb/pkg/server.(*clientConn).writeChunks
/workspace/source/tidb/pkg/server/conn.go:2339
github.com/pingcap/tidb/pkg/server.(*clientConn).writeResultSet
/workspace/source/tidb/pkg/server/conn.go:2282
github.com/pingcap/tidb/pkg/server.(*clientConn).handleStmt
/workspace/source/tidb/pkg/server/conn.go:2085
github.com/pingcap/tidb/pkg/server.(*clientConn).handleQuery
/workspace/source/tidb/pkg/server/conn.go:1794
github.com/pingcap/tidb/pkg/server.(*clientConn).dispatch
/workspace/source/tidb/pkg/server/conn.go:1384
github.com/pingcap/tidb/pkg/server.(*clientConn).Run
/workspace/source/tidb/pkg/server/conn.go:1153
github.com/pingcap/tidb/pkg/server.(*Server).onConn
/workspace/source/tidb/pkg/server/server.go:740
runtime.goexit
/root/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.23.11.linux-amd64/src/runtime/asm_amd64.s:1700
```

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

```
Release Version: v9.0.0-beta.2.pre-118-g6ec4810
Edition: Community
Git Commit Hash: 6ec48101cae75529682a0f9fa8c16cbc42f246d3
Git Branch: HEAD
UTC Build Time: 2025-07-16 11:00:02
GoVersion: go1.23.11
Race Enabled: false
Check Table Before Drop: false
Store: unistore
Kernel Type: Classic
```

Reproduce the ADMIN CHECK INDEX sequence on the nightly Docker image, then inspect pkg/util/chunk/codec.go around decodeColumn and pkg/executor/admin.go around CheckIndexRangeExec.Next. Trace how the index result is decoded and add a regression test for the reported JSON multi-valued index case. Done means the statement completes without the runtime error.

Written by the indexing model from the issue text.