ADMIN CHECK reports false data inconsistency for partial multi-valued indexes

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
DROP TABLE IF EXISTS t;

CREATE TABLE t (
pk INT PRIMARY KEY,
a JSON,
flag INT,
INDEX idx ((CAST(a AS SIGNED ARRAY))) WHERE flag = 1
);

INSERT INTO t VALUES
(0, '[0,1,2]', 0),
(1, '[1,2,3]', 1);

ADMIN CHECK TABLE t;
ADMIN CHECK INDEX t idx;
```

The row with pk = 0 does not satisfy flag = 1, so it should not have any entries in idx.

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

Both `ADMIN CHECK TABLE t` and `ADMIN CHECK INDEX t idx` should complete successfully.

Rows that do not satisfy the partial index condition are not expected to have index entries and should be skipped by the record-to-index consistency check.

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

Both commands incorrectly report data inconsistency for the row excluded by the partial index condition:

```
ERROR 8223 (HY000): data inconsistency in table: t, index: idx,
handle: 0, index-values:"" != record-values:
"handle: 0, values: [KindMysqlJSON [0, 1, 2]]"
```

The record-side check calls Index.Exist for every table row without first evaluating the partial index condition, causing a false-positive inconsistency report.

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

The issue was reproduced on TiDB master:

```
Git Commit Hash: 66f307fab7ddfd1a9861715cd5533a04ebe2379d
Git Branch: master
```