ast/formatter: NOT EXISTS (subquery) renders as NOT (EXISTS (...) NOT ), which does not re-parse #601

Summary

The parser represents NOT EXISTS (subquery) as a BinaryExpression{Operator: "NOT", Not: true, Left: ExistsExpression, Right: nil}. BinaryExpression.writeSQL has no case for this unary-NOT shape. It falls into the generic Not branch and emits NOT (<left> NOT <nil>). ast.SQL(), pkg/formatter (compact and readable) and formatter.FormatString therefore all produce SQL that fails to parse.

Reproduction

tree, _ := gosqlx.Parse("SELECT * FROM t WHERE NOT EXISTS (SELECT 1 FROM u)")
fmt.Println(tree.SQL())
fmt.Println(formatter.FormatAST(tree, ast.CompactStyle()))

Actual (main 61ca6c8):

SELECT * FROM t WHERE NOT (EXISTS (SELECT 1 FROM u) NOT )

Re-parsing fails with Error E2002 at line 1, column 53: expected ), got NOT. formatter.FormatString gives SELECT *\nFROM t\nWHERE NOT (EXISTS (SELECT 1 FROM u) NOT );, which fails the same way. The same happens inside larger predicates (... NOT EXISTS (...) AND a = 1).

Expected: SELECT * FROM t WHERE NOT EXISTS (SELECT 1 FROM u).

Notes

• pkg/sql/ast/sql.go:313 (BinaryExpression.writeSQL): the if b.Not block at ~line 347 writes NOT ( Left op Right ) with no nil-Right guard. Add a branch for Right == nil (operator NOT) that writes NOT followed by Left. pkg/formatter reaches this code through exprSQL and SQL(), so the one fix covers both.
• The CLI formatter's version of this bug ([BUG] gosqlx format panics (nil pointer) on NOT EXISTS; CLI formatter lacks ExistsExpression case #587) is fixed: gosqlx format now prints WHERE NOT EXISTS (...). [BUG] gosqlx format panics (nil pointer) on NOT EXISTS; CLI formatter lacks ExistsExpression case #587 stated that the library renderers were fine, but they are not.
• Add a round-trip test for NOT EXISTS through SQL(), FormatAST and FormatString.

Found during review of #591.