When timezone param of unix_timestamp is CST, ambiguous zone will lead different result.

### Backend

VL (Velox)

### Bug description

example:
```
set spark.sql.optimizer.excludedRules=org.apache.spark.sql.catalyst.optimizer.ConstantFolding;
select from_unixtime(
unix_timestamp('Sun Jun 14 20:00:05 CST 2026', 'EEE MMM dd HH:mm:Ss z yyyy'),
'yyyy-MM-dd HH:mm:ss');
```

vanilla: 2026-06-14 20:00:05
gluten: 2026-06-15 09:00:05

### Gluten version

1.6.0
### Spark version

3.5.8

### Spark configurations

spark.sql.legacy.timeParserPolicy=LEGACY

### System information

_No response_

### Relevant logs

```bash

```

Start by reproducing the unix_timestamp/from_unixtime query with spark.sql.legacy.timeParserPolicy=LEGACY on the VL backend and compare it with vanilla Spark. Trace how the CST timezone is parsed and converted, then verify that the result matches vanilla Spark for the supplied query.

Written by the indexing model from the issue text.