Support Oracle XMLSERIALIZE CONTENT and DOCUMENT forms

[minleejae]

Fixes #1564

Changes

• Extend XMLSerializeExpr with Oracle CONTENT/DOCUMENT value expressions, optional datatype, ENCODING, VERSION, indentation, and HIDE/SHOW DEFAULTS.
• Keep the existing XMLAGG(XMLTEXT(...)) form and its expression/order-by getters compatible. Omitted Oracle options remain omitted in the AST and output.
• Share rendering between toString and ExpressionDeParser, and share child discovery across expression, table-name, and validation visitors.
• Fix the absent-ORDER-BY visitor null dereference and traverse serialized subqueries/options. Honor custom expression deparsers for legacy ORDER BY expressions with visitor context.
• Add context-aware OrderByDeParser overloads while preserving existing subclass customization points.

Verification

• Full Gradle check (tests, Checkstyle, PMD, Spotless).
• Maven clean verify with Java 17.
• Oracle and legacy round-trips, malformed options, omission/AST inspection, visitor and custom deparser regressions, and table discovery in serialized subqueries.

Reference: Oracle XMLSERIALIZE.