feat(parser,ast): preserve parenthesized expressions (fix #519)

[basili4-1982]

Fixes #519.

Summary

Parenthesized expressions in WHERE (and anywhere an expression appears) were
silently dropped during parsing, so group-precedence was lost on re-render:

input  : SELECT * FROM t WHERE (a = 1 OR b = 2) AND c = 3
current: SELECT * FROM t WHERE a = 1 OR b = 2 AND c = 3

This silently changes the meaning of the predicate.

Root cause

parsePrimaryExpression parsed (expr) and returned the bare expr, discarding
the grouping. There was no AST node to represent parentheses, so round-trip
could not preserve them.

Changes

• pkg/sql/ast/ast_expressions.go: new ParenthesizedExpression node
  (Expr, Pos).
• pkg/sql/ast/sql.go: (*ParenthesizedExpression).SQL() emits (...).
• pkg/sql/parser/expressions_literal.go: single parenthesized primary
  expressions now wrap in a ParenthesizedExpression (tuple (a, b, c)
  and subquery (SELECT ...) paths unchanged).
• pkg/sql/ast/pool_expression_release.go: release child expr; the node itself
  needs no dedicated pool.
• pkg/sql/ast/roundtrip_test.go: grouped-OR and nested-parens cases.
• pkg/sql/parser/parenthesized_expression_test.go: regression tests.

Test plan

• New regression tests TestParenthesizedExpressionRoundtrip pass
• go test -short ./pkg/sql/ast/ ./pkg/sql/parser/ ./pkg/gosqlx/ ./pkg/formatter/ green
• gosqlx.Format(...) preserves parentheses
• Nested parens ((a AND b) OR c) AND d round-trip unchanged

Note: TestPerformanceRegression fails in this environment against its
baseline (5 cases) — unrelated to this change, reproduced on unmodified main.

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[basili4-1982]

Проверено, готово к мержу.

• mergeable: MERGEABLE — без конфликтов с main.
• Тесты зелёные: go test -short ./pkg/sql/ast/ ./pkg/sql/parser/ ./pkg/formatter/ ./pkg/gosqlx/ (включая TestParenthesizedExpressionRoundtrip), gosqlx.Format тоже сохраняет скобки.
• Единственный красный check — Vercel (Authorization required to deploy): деплой форка, не связан с кодом. Apply labels и GitGuardian проходят.

Прошу ревью/мерж. Фикс закрывает #519 (скобки в WHERE не теряются).