parser: EXPLAIN FORMAT='hint' restores hint-level QB_NAME inside nested LEADING

## Bug Report

### 1. Minimal reproduce step (Required)

Create three tables:

```sql
DROP TABLE IF EXISTS t1, t2, t3;

CREATE TABLE t1 (a INT);
CREATE TABLE t2 (a INT);
CREATE TABLE t3 (a INT);
```

The original hint can be parsed and used without an optimizer hint syntax warning:

```sql
SELECT /*+ QB_NAME(qb), LEADING(@qb (t1,t2),t3) */
t1.a, t2.a, t3.a
FROM t1
JOIN t2 ON t1.a = t2.a
JOIN t3 ON t2.a = t3.a;

SHOW WARNINGS;
```

Run `EXPLAIN FORMAT='hint'` for the same query:

```sql
EXPLAIN FORMAT='hint'
SELECT /*+ QB_NAME(qb), LEADING(@qb (t1,t2),t3) */
t1.a, t2.a, t3.a
FROM t1
JOIN t2 ON t1.a = t2.a
JOIN t3 ON t2.a = t3.a;
```

Copy the returned hint into the original query and run it again, for example:

```sql
SELECT /*+ qb_name(`qb`), leading((@`qb` `t1`, `t2`), `t3`) */
t1.a, t2.a, t3.a
FROM t1
JOIN t2 ON t1.a = t2.a
JOIN t3 ON t2.a = t3.a;

SHOW WARNINGS;
```

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

`EXPLAIN FORMAT='hint'` should return a syntactically valid, replayable hint. The hint-level query block name should remain outside the nested list, for example:

```text
leading(@`qb` (`t1`, `t2`), `t3`)
```

Replaying the returned hint should not produce an optimizer hint syntax warning.

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

`EXPLAIN FORMAT='hint'` returns the hint-level query block name inside the first nested list, for example:

```text
qb_name(`qb`), leading((@`qb` `t1`, `t2`), `t3`)
```

Replaying this output produces an optimizer hint syntax error (1064), so the generated hint is not replayable and the `LEADING` constraint is ignored.

The issue is triggered when a `LEADING` hint has a hint-level `QB_NAME` and its first element is a parenthesized list, such as `LEADING(@qb (t1,t2),t3)`. Flat forms such as `LEADING(@qb t1,t2)` are not affected.

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

Observed on `8.0.11-TiDB-v8.5.7` (release-8.5).

Related: #63253 and #68867.

Start by running the provided three-table SQL reproduction and compare the original LEADING hint with EXPLAIN FORMAT='hint' output. Trace the parser and hint-restoration path for nested LEADING lists; done means the generated hint keeps @qb outside the nested list and replays without optimizer hint syntax warnings.

Written by the indexing model from the issue text.