ROLLUP/CUBE/GROUPING SETS: an element starting with ( is always read as a sublist ((SELECT 1) rejected, ((b)) displayed as (b)) #2806

Each element of ROLLUP, CUBE and GROUPING SETS treats a leading ( as its own sublist delimiter, which breaks parenthesized expressions:

SELECT a FROM t GROUP BY ROLLUP (a, (SELECT 1))
  -> sql parser error: Expected: ), found: 1 at Line: 1, Column: 45
SELECT a FROM t GROUP BY CUBE (a, (SELECT 1))
SELECT a FROM t GROUP BY GROUPING SETS (a, (SELECT 1))
  -> same error

SELECT a FROM t GROUP BY ROLLUP (a, ((b)))
  -> displays as: SELECT a FROM t GROUP BY ROLLUP (a, (b))
  -> reparses as ROLLUP (a, b): element [Nested(b)] becomes [b]

PostgreSQL 18 runs SELECT a FROM t GROUP BY ROLLUP (a, (SELECT 1)). Its docs: "The individual elements of a CUBE or ROLLUP clause may be either individual expressions, or sublists of elements in parentheses" (https://www.postgresql.org/docs/current/queries-table-expressions.html#QUERIES-GROUPING-SETS). The second case changes only the AST, since PostgreSQL treats ((b)) and (b) the same.

parse_group_by_expr parses each element with parse_tuple(true, true), which consumes the ( of (SELECT 1) and then reads SELECT as an identifier. Display for Rollup/Cube writes a one-element set without its own parentheses, so [Nested(b)] prints as (b).

Tested with PostgreSqlDialect on main ce12f1f (also 0.63.0 and 0.58.0).

Found with a property-based fuzzer (round trip check); I used AI assistance for triage and verified every case above.