test: cover a cast inside an aggregate's argument

[prql-bot]

#3534 reported that sum (artist_id | as int) inside aggregate failed to type-check (function std.sum, param `column` expected type `array`, but found type `scalar` ), while the same cast moved into a preceding derive worked. On current main the issue's query compiles to exactly the SQL it asked for:

SELECT
  COALESCE(SUM(CAST(artist_id AS int)), 0) AS total_int,
  COALESCE(SUM(CAST(artist_id AS int)), 0) AS total_cast
FROM
  artists

The likely fix is #5111, which dropped the <scalar> return type from std.as (read from that diff, not bisected). No test passes a cast to an aggregate, so this adds test_aggregate_of_cast to prqlc/prqlc/tests/integration/sql.rs with the issue's query, next to the similar test_aggregate_of_expression from #6400.

Test-only change, so no changelog entry.

Closes #3534