next版本 明细表冻结列之后，无法合并冻结列的单元格

| Package | Version |
| -------------- | ------- |
| @antv/s2 | next |
| @antv/s2-react | |
| @antv/s2-vue | |

### Sheet Type

- [ ] PivotSheet
- [x] TableSheet
- [ ] GridAnalysisSheet
- [ ] StrategySheet
- [ ] EditableSheet

### 🖋 Description

明细表冻结列之后，无法合并冻结列中的单元格。

### 🔗 Reproduce Link
使用官网的demo，即可复现
https://s2.antv.antgroup.com/examples/layout/custom-header-group/#custom-table-col-header

```
const s2Options: S2Options = {
width: 600,
height: 480,
seriesNumber: {
enable: true
},
mergedCellsInfo: [
[
{ colIndex: 1, rowIndex: 6, showText: true },
{ colIndex: 1, rowIndex: 7 },
{ colIndex: 2, rowIndex: 6 },
{ colIndex: 2, rowIndex: 7 },
{ colIndex: 3, rowIndex: 6 },
{ colIndex: 3, rowIndex: 7 },
],
],
frozen: {
// 列头冻结数量
colCount: 2,
},
};
```

### 😊 Expected Behavior

### 😅 Current Behavior
当前配置无法实现

Start with the official custom-table-col-header demo linked in the issue and reproduce the TableSheet configuration using mergedCellsInfo with frozen.colCount set to 2. Trace how these options interact, then verify that the specified cells can be merged within frozen columns while preserving the expected display shown in the issue.

Written by the indexing model from the issue text.