`all_varchar = true` converts boolean values to numbers before returning them as strings

### Problem Description

I'm trying to read the data from some excel files as strings. However, excel extension is coercing the boolean values to their numeric representations before returning them as strings. This leads to some interesting quirks when I'm dealing with mixed values in a column. The following reproduction dramatizes the ambiguity caused by this issue.

### How to reproduce

Run the following query on this file: [wb.xlsx](https://github.com/user-attachments/files/24196107/wb.xlsx).

```sql
select * from read_xlsx('./wb.xlsx', header = true, all_varchar = true);
```

### Expected behavior

| Property | Value |
|--------|--------|
| available | TRUE |
| units | 1 |
| returnable | FALSE |
| shipping_charges | 0 |

### Current behavior

| Property | Value |
|--------|--------|
| available | 1 |
| units | 1 |
| returnable | 0 |
| shipping_charges | 0 |

### Version Information

```sh
DuckDB v1.4.2 (Andium) 68d7555f68
```

Start by running the supplied read_xlsx query against wb.xlsx with all_varchar = true and trace how boolean cells are converted. Done when boolean values are returned as TRUE and FALSE strings while numeric values remain unchanged, and the reproduction behaves as described.

Written by the indexing model from the issue text.