fix feature-service bug

[yu-han-yang]

No description provided.

[yu-han-yang]

@man-zhang

[whytomato]

Problem

SqlNameContext could not correctly process aliases assigned to parenthesized subqueries, particularly when the subquery contained a set operation such as UNION ALL.

For example:

SELECT derived.id
FROM (
    SELECT id FROM constraint_requires
    UNION ALL
    SELECT id FROM constraint_excludes
) derived
WHERE derived.id = 2

The previous implementation called ParenthesedSelect#getPlainSelect(). However, a UNION query is represented by a SetOperationList, so getPlainSelect() can return null, resulting in a NullPointerException.

The alias was also read from the inner FROM item, although derived belongs to the enclosing ParenthesedSelect.

Fix

The alias is now read directly from the ParenthesedSelect.

Parenthesized subqueries are represented as UNNAMED_TABLE, and their aliases are mapped to that value. This avoids incorrectly assuming that every parenthesized SELECT contains a PlainSelect and allows qualified columns such as derived.id to be resolved correctly.