fix: SQLDeleteStatement.accept0 missing from/using children (#6563)

[daguimu]

Problem

The base SQLDeleteStatement.accept0 walked only with / tableSource / where, silently skipping the from and using sub-trees. As a result, dialects that fall back to the base implementation (DB2, Snowflake, etc.) cannot be fully traversed by visitors that rely on the default tree walk - any AST node reachable only via DELETE ... FROM <ts> or DELETE ... USING <ts> was invisible to AST consumers.

getChildren() had the same omission, and setUsing() failed to wire the parent pointer (setFrom already does), breaking parent-chain traversal for that branch.

Reported in #6563 (DELETE statement visitor cannot reach the FROM sub-tree when modifying table names).

Root Cause

Compare with MySqlDeleteStatement.accept0, which correctly walks with, tableSource, where, from, using, orderBy, limit. The base class - used directly by Snowflake, DB2 and any future dialect that does not override - was missing from and using. Snowflake and PG/Oscar parsers populate the using slot via setUsing(), but no parent was assigned, so even reverse-traversal would not work.

Fix

In SQLDeleteStatement:

• accept0: walk from and using in addition to the previously-visited children.
• getChildren: include from and using.
• setUsing: wire the parent pointer, mirroring setFrom.

No changes to dialect-specific subclasses - they already override accept0 and were unaffected.

Tests Added

core/src/test/java/com/alibaba/druid/sql/issues/Issue6563.java:

• test_snowflake_delete_using_visited - parses DELETE FROM t1 USING t_using_only WHERE ..., walks the AST with a generic identifier collector, and asserts t_using_only (referenced ONLY through the USING clause) is reached.
• test_db2_delete_from_visited - parses DELETE FROM t FROM t_from_only WHERE ... for DB2, asserts t_from_only (reachable only through the secondary FROM) is visited.
• test_getChildren_includes_from_and_using - structural test on the AST API.
• test_setUsing_sets_parent - verifies the parent pointer is wired so reverse traversal works.

All four tests fail on master (4 failures, no errors) and pass with the fix.

Full mvn test for the core module: 4222 tests, 0 failures, 0 errors.

Fixes #6563