Sort key over get_field(named_struct(...), f) is evaluated against the materialized struct instead of the base column

Follow-up from #22239. For a query that sorts by a struct field, where the table is `WITH ORDER (a + b)` so a `SortExec` is genuinely required:

```sql
EXPLAIN SELECT named_struct('a', a, 'b', b) AS s FROM ordered ORDER BY s['a'];
```

**Plan before #22239** — `get_field` sort key extracted into a flat scan column, recovery projection hides it:

```
01)ProjectionExec: expr=[s@0 as s]
02)--SortExec: expr=[__datafusion_extracted_1@1 ASC NULLS LAST], preserve_partitioning=[false]
03)----DataSourceExec: projection=[named_struct(a, a@0, b, b@1) as s, get_field(named_struct(a, a@0, b, b@1), a) as __datafusion_extracted_1], ...
```

**Plan after #22239** — simpler, but the sort key is a struct-field extraction over the materialized struct:

```
01)SortExec: expr=[get_field(s@0, a) ASC NULLS LAST], preserve_partitioning=[false]
02)--DataSourceExec: projection=[named_struct(a, a@0, b, b@1) as s], ...
```

**Ideal plan** — sort on the base column, build the struct afterward:

```
01)ProjectionExec: expr=[named_struct(a, a@0, b, b@1) as s]
02)--SortExec: expr=[a@0 ASC NULLS LAST]
03)----DataSourceExec: projection=[a, b]
```

Start by reproducing the provided EXPLAIN query against the ordered table and compare the current plan with the ideal plan. Trace how the sort key for get_field(named_struct(...), 'a') is extracted and when the struct is materialized; done means SortExec orders by the base column and the projection builds the struct afterward.

Written by the indexing model from the issue text.