Fix MySQL AST conversion dropping NOT from NOT EXISTS expressions.

the affected syntax in two different ways:

• NOT (EXISTS (...)) as a unary NOT operation
• NOT EXISTS (...) through ExistsSubqueryExpr.Not

The Dolphin converter did not handle either representation correctly. In the parenthesized case, the entire WHERE expression could be dropped, including nested sqlc.arg parameters. In the direct case, the query was preserved but the NOT operator was lost.

This change:

• converts unary logical NOT expressions to ast.BoolExpr
• preserves ExistsSubqueryExpr.Not
• adds regression coverage for both forms
• verifies that nested named parameters remain present
  Missing sqlc.arg in SQL UPDATE query with EXISTS subquery #4512