[BUG] Mock request lookup by apiId returns an arbitrary record when duplicates exist

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/shenyu/issues) and found no similar issues.

### Apache ShenYu Component

shenyu-admin

### What happened

`MockRequestRecordServiceImpl.queryByApiId(...)` returns the first row from `selectByQuery(...)`:

```java
MockRequestRecordQuery mockRequestRecordQuery = new MockRequestRecordQuery();
mockRequestRecordQuery.setApiId(apiId);
List mockRequestRecordDOList = mockRequestRecordMapper.selectByQuery(mockRequestRecordQuery);
return mockRequestRecordDOList.isEmpty()
? new MockRequestRecordVO()
: MockRequestRecordVO.buildMockRequestRecordVO(mockRequestRecordDOList.get(0));
```

But the mapper query has no `ORDER BY`:

```xml

SELECT ...
FROM mock_request_record


and api_id = #{apiId, jdbcType=VARCHAR}

...

```

and the table schema only defines `id` as the primary key. There is no uniqueness constraint on `api_id`:

```sql
CREATE TABLE `mock_request_record` (
`id` varchar(128) NOT NULL,
`api_id` varchar(128) NOT NULL,
...
PRIMARY KEY (`id`)
)
```

If multiple mock request records exist for the same `apiId`, the returned record depends on the database execution plan and can change unpredictably.

### Expected behavior

`queryByApiId(...)` should be deterministic. It should either enforce a unique mock request record per `apiId`, add a deterministic ordering and selection rule, or return all matching records instead of silently picking the first unordered row.

### How to reproduce

1. Insert or create two `mock_request_record` rows with the same `api_id` and different request data.
2. Call the admin API path that uses `MockRequestRecordServiceImpl.queryByApiId(apiId)`.
3. The service returns `selectByQuery(...).get(0)` without any SQL ordering or uniqueness guarantee.
4. Which mock record is returned is not stable across database plans/storage state.

### Debug logs

_No response_

### Environment

Current `master` branch.

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

Start at MockRequestRecordServiceImpl.queryByApiId(...) and the mapper's selectByQuery XML, then reproduce the behavior with duplicate api_id rows. The issue presents several possible fixes but does not select one; done requires an agreed deterministic contract and a regression test proving duplicate lookups no longer return an arbitrary record.

Written by the indexing model from the issue text.