[SQL] Add CASE WHEN equivalent for the DECODE function

Currently, the `decode` function is used only for decoding strings. However, a `CASE WHEN` equivalent has been available in the Spark SQL API since version 3.2.0

https://spark.apache.org/docs/latest/api/sql/string-functions/#decode

```
> SELECT decode(encode('abc', 'utf-8'), 'utf-8');
abc
> SELECT decode(6, 1, 'Southlake', 2, 'San Francisco', 3, 'New Jersey', 4, 'Seattle', 'Non domestic');
Non domestic
```

Start with the linked Spark SQL DECODE documentation and the existing DECODE SQL entry point, comparing the string-decoding examples with the requested CASE WHEN form. Trace where SQL function behavior is covered by tests, then verify that the new conditional form works without regressing the existing examples.

Written by the indexing model from the issue text.