[VL][1.5] Result mismatch of `from_unixtime` function

### Backend

VL (Velox)

### Bug description

sql:

```
select from_unixtime(1549787517000, 'yyyy-MM-dd');
```

spark result: `+51080-10-23`

gluten result: `51080-10-23`

### Gluten version

Gluten-1.5

### Spark version

Spark-3.5.x

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```

Start by running the reported SQL query with Spark 3.5.x and Gluten 1.5 using the VL backend, then compare the formatted results. Trace the from_unixtime implementation and its result-formatting path; done means Gluten produces the same output as Spark, including the leading sign.

Written by the indexing model from the issue text.