Using filter + within group is not supported

Here is a minimally reproducing query:

```sql
select
e.instance_id
-- this next one is fine
, percentile_cont(0.75) within group (order by e.running_time) / 1000.0 as p75_time
-- this is also fine
, avg(e.running_time) filter (where e.error = '') / 1000.0 as avg_success_time
-- this one causes a parse error
, percentile_cont(0.75) within group (order by e.running_time) filter (where e.error = '') / 1000.0 as p75_success_time
from execution e
group by 1
```

Begin with the minimal SQL reproduction and the Clojure wrapper's parsing entry point; compare the supported standalone WITHIN GROUP and FILTER forms. Done means the combined expression parses successfully, with a regression test covering the query.

Written by the indexing model from the issue text.