sql: ORDER BY / LIMIT on the outer layer of a two-layer query is rejected #891

Problem

A two-layer query with a plain (non-BY) ORDER BY / LIMIT on its outer layer is rejected. SQLPatternMatcher::nested_query_to_pattern returns QueryError::InvalidNestedQuery (ORDER BY / LIMIT on the outer layer is not supported), and the planner fails with a SqlParse error.

SELECT job, SUM(v) AS s FROM (
  SELECT job, instance, SUM(value) AS v FROM data
  WHERE time BETWEEN DATEADD(s, -1, NOW()) AND NOW()
  GROUP BY job, instance
) GROUP BY job
ORDER BY s DESC LIMIT 5

Equivalent PromQL: topk(5, sum by (job) (sum by (job, instance) (data)))

The same query without ORDER BY / LIMIT is supported (#819): the inner layer is planned as a single-level query and the outer SUM runs as a query-time aggregation. Ordering and truncating the outer result is not wired up: it is rejected rather than dropped, since top-k detection on a nested query only sees its inner layer.

Related: #818 (LIMIT k BY), #820 (outer SELECT without an aggregate), #821 (ORDER BY / LIMIT inside the subquery).

Reproduce

cargo test -p sql_utilities test_nested_query_rejects_outer_order_by_and_limit