Add sqllogictest coverage for UDWF return types in information_schema

### Context

In #20079, the `information_schema` was updated to use `return_field_from_args` / `return_field` / `WindowUDFFieldArgs::field` instead of the older `return_type` API for UDFs, UDAFs, and UDWFs.

As noted in [this review comment](https://github.com/apache/datafusion/pull/20079#discussion_r2749229459), it would be good to add sqllogictest coverage to verify that `information_schema.routines` correctly reports return types for window UDFs (the `get_udwf_args_and_return_types` path).

### What needs to be done

Add a sqllogictest (in `test_files/information_schema.slt` or a new file) that:

1. Registers a window UDF (or uses a built-in one)
2. Queries `information_schema.routines` filtering for that window function
3. Asserts the return type column is populated correctly (previously it was always `NULL`)

The relevant code is in [`datafusion/catalog/src/information_schema.rs`](https://github.com/apache/datafusion/blob/main/datafusion/catalog/src/information_schema.rs) around the `get_udwf_args_and_return_types` function.

### Willingness to submit a PR

Yes, but as this can potentially be a good newcomer task i'm up to leave for someone else

Start with test_files/information_schema.slt and inspect existing information_schema sqllogictests, then read datafusion/catalog/src/information_schema.rs around get_udwf_args_and_return_types. Add coverage that registers or uses a window UDF, queries information_schema.routines for it, and asserts the return type is populated correctly rather than NULL.

Written by the indexing model from the issue text.