Complete traversal of structured DDL children - #2738

Purpose

Make visitor-based analysis and validation reach structured DDL children while preserving the distinction between stored schema definitions and executed statements.

Current design (As-is)

Several DDL nodes already expose structured expressions and tables, but the default statement visitor does not traverse them. INSERT loses the caller's context when visiting its SELECT, and VALUES traversal omits WITH items, ordering and pagination children.

Target design (To-be)

Connect relation and policy alterations, statistics, foreign-data objects, rules, notifications and collations to the configured child visitors. Preserve the original context through rule actions, INSERT and VALUES descendants. Extend expression validation for relation alterations, policies and statistics.

Keep stored rule actions and policy predicates classified as schema definitions in feature analysis. Preserve existing parser result classes and public signatures; grammar and dialect acceptance are unchanged.

Validation

After merging master at 40ed289b, the full Java 17 Gradle check passed: 9,857 tests total, 9,832 passed, 25 skipped, zero failures. Grammar choice-conflict checks, Spotless, Checkstyle, PMD, SpotBugs and coverage verification passed. Regression tests cover child traversal, context propagation, validation and feature classification.

GitHub CI at c13f7ee9 also passes: Codacy, grammar conflicts, and Gradle/Maven checks on Linux, macOS and Windows.