Remove nested SQL query support at the sql_utilities matcher level #499

Part of #497.

Nested SQL aggregations are currently matched and planned but executed incorrectly (outer aggregation silently discarded). Remove matching/support for them at the AST matcher level in asap-common/dependencies/rs/sql_utilities/src/ast_matching/sqlpattern_matcher.rs, and add a new QueryError::NestedQueryUnsupported variant (matcher currently defines QueryError at line 17) to reject them explicitly instead of mis-executing.

Mirrors #496's PromQL-side nested query issue — keep the error shape consistent with that fix.