Nested `unnest` of a struct reports an internal error ("Assertion failed: struct_allowed") instead of a planning error

### Describe the bug

`unnest` on a struct is only supported as the top-level select expression. Using it anywhere else hits `assert_or_internal_err!(struct_allowed, ...)` in `RecursiveUnnestRewriter::transform` (`datafusion/sql/src/utils.rs`), so the user gets an *internal* error that asks them to file a bug, even though the condition is entirely determined by the shape of their query.

### To Reproduce

```sql
SELECT unnest(unnest(struct([1])));
SELECT unnest(struct(1))['c0'];
```

```text
Internal error: Assertion failed: struct_allowed: unnest on struct can only be applied at the root level of select expression.
This issue was likely caused by a bug in DataFusion's code. Please help us to resolve this by filing a bug report in our issue tracker: https://github.com/apache/datafusion/issues
```

`unnest.slt` even has a test (`select arrow_typeof(unnest(column5)) from unnest_table`) that pins this internal-error text.

### Expected behavior

```text
Error during planning: unnest on struct can only be applied at the root level of select expression
```

### Additional context

Found while running a corpus of unusual `unnest` expressions against `datafusion-cli`.

Start in datafusion/sql/src/utils.rs at RecursiveUnnestRewriter::transform, where struct_allowed currently produces the internal error. Review the unnest.slt coverage, including the arrow_typeof(unnest(column5)) case, and run the relevant SQL tests. Done means nested struct unnest expressions report the stated planning error instead of an internal error.

Written by the indexing model from the issue text.