`AT TIME ZONE '+05:30'` uses the opposite sign convention from PostgreSQL

### Describe the bug

`AT TIME ZONE` with a fixed-offset *string* uses the opposite sign convention from PostgreSQL.

DataFusion 55.0.0:

```sql
SELECT arrow_cast(TIMESTAMP '2024-01-01 12:00:00','Timestamp(Second, Some("UTC"))')
AT TIME ZONE '+05:30';
-- 2024-01-01T17:30:00+05:30
```

PostgreSQL 17.11:

```sql
SET TimeZone='UTC';
SELECT '2024-01-01T12:00:00Z'::timestamptz AT TIME ZONE '+05:30' AS as_string,
'2024-01-01T12:00:00Z'::timestamptz AT TIME ZONE INTERVAL '05:30' AS as_interval;

as_string | as_interval
---------------------+---------------------
2024-01-01 06:30:00 | 2024-01-01 17:30:00
```

PostgreSQL treats the *string* `'+05:30'` as POSIX-style, where the sign is west-positive, giving `06:30`. It treats the *interval* `INTERVAL '05:30'` as east-positive, giving `17:30`. DataFusion's string form matches PostgreSQL's interval form, i.e. it uses the ISO/RFC-3339 convention that Arrow uses everywhere else.

DuckDB 1.5.2 rejects `'+05:30'` in `AT TIME ZONE` outright rather than picking a convention.

### To Reproduce

The queries above.

### Expected behavior

Unclear, deliberately. DataFusion's answer is arguably the better one — it is self-consistent with how `Timestamp(_, Some("+05:30"))` is interpreted everywhere else in Arrow, and PostgreSQL's own string/interval split is a well-known wart. But it means a query copied from PostgreSQL silently returns an instant 11 hours away, with no error.

Filing it so the divergence is on record and the choice is deliberate. Reasonable outcomes: keep the behaviour and document it, or reject bare fixed-offset strings in `AT TIME ZONE` as DuckDB does and require `INTERVAL`.

This is pre-existing and orthogonal to #25165, which changes only the result *type* of `AT TIME ZONE` on an already-aware value.

Start by running the supplied DataFusion queries and comparing their results with the PostgreSQL examples for string and interval offsets. The work is complete when the project has made and recorded a deliberate choice: document the existing behavior, reject bare fixed-offset strings, or change the behavior with corresponding coverage.

Written by the indexing model from the issue text.