parser: keep parenthesized set-op operand and bind trailing ORDER BY/LIMIT to the whole set

[chiradip]

What

Two set-operation parsing defects (parser robustness audit):

1. Parenthesized right operand silently dropped. parse_setop_operand only accepted a bare SELECT keyword, so SELECT 1 UNION (SELECT 2) returned just SELECT 1 and abandoned UNION (SELECT 2) as trailing tokens. It now also accepts a parenthesized query block ( <query> ) — which may itself be a set operation and keeps its own ORDER BY/LIMIT — consuming the matching ).

2. Trailing ORDER BY/LIMIT bound to the right branch, not the whole set. The right operand (parsed via parse_select_stmt in in_setop_rhs_ mode) consumed the ORDER BY/LIMIT before the set-op-rhs early return, so SELECT 1 UNION SELECT 2 ORDER BY 1 attached ORDER BY to the RHS SELECT and then failed validation. The ORDER BY/LIMIT parsing is factored into a lambda, skipped for bare set-op operands, and re-run once after folding so it attaches to the combined set-op node. A plain SELECT is unaffected (consumes its clauses on the first pass; the post-fold pass is a no-op).

Before / after

• SELECT 1 UNION (SELECT 2) — before: SelectStmt(1) (4 trailing tokens); after: UnionStmt → [SelectStmt(1), SelectStmt(2)].
• SELECT 1 UNION SELECT 2 ORDER BY 1 — before: PARSE ERROR: validation failed; after: UnionStmt → [SelectStmt(1), SelectStmt(2), OrderByClause].

Known remaining gap (follow-up)

A leading parenthesized operand (SELECT 1) UNION (SELECT 2) still errors — parse_select_stmt's entry point doesn't yet accept a leading (. That's a separate, more invasive change and is tracked as a follow-up; this PR is scoped to the two confirmed silent-drop / mis-parse findings.

Tests

test_precedence_regression: parenthesized right operand keeps both branches; trailing ORDER BY and ORDER BY + LIMIT attach to the UNION node; a plain SELECT still attaches its own ORDER BY/LIMIT (regression guard). Full suite 37/37 green.

Completes the parser-layer set-operation robustness (CASE #46, IS DISTINCT FROM #47, IN #48 already up/merged).