octet_length(char) doesn't behave as expected.

### Describe the bug

Hi there,

``select octet_length(char)`` doesn't behave as expected.

In the example below - in Postgres (``text`` column) the below SQL would return 25 as the result however in DataFusion (``StringArray`` column ) it just returns the original length of the values in ``n_name``.

```sql
select octet_length(n_name::char(25)) from nation;
```

I'm not sure if this is because the cast to a ``char(25)`` is not behaving as expected

### To Reproduce

create a StringColumn in DF, populate it with some values and run the following SQL on it.

```sql
select octet_length(n_name::char(25)) from nation;
```

### Expected behavior

Should return 25, but it returns the length of each of the values in the column.

### Additional context

_No response_

Inizia con la riproduzione SQL usando un DataFusion StringArray e confronta octet_length(n_name::char(25)) con il risultato di PostgreSQL. Traccia il cast a char(25) e il comportamento di octet_length; quindi aggiungi una copertura che mostri che l’espressione restituisce 25 per i valori riprodotti.

Scritto dal modello di indicizzazione a partire dal testo della issue.