Restore Alias branch in expandProjectFields for IN/EXISTS subqueries

PR opensearch-project#5448 (parent commit b10d541) introduced SQL IN/EXISTS subquery
support that produces a Project AST whose project list contains Alias
expressions. The 'Lower structured search predicates' commit
(c18b11e) on this PR accidentally dropped the case Alias branch from
CalciteRelNodeVisitor.expandProjectFields, so the subquery's project
list hit the default arm and threw 'Unexpected expression type in
project list: Alias'.

Restoring the one-line branch fixes UnifiedQueryPlannerSqlV2Test
testInSubquery / testNotInSubquery / testExistsSubquery /
testNotExistsSubquery (the four macOS unit-test failures on this PR).

Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java

@@ -730,6 +730,7 @@ private List<RexNode> expandProjectFields(
               .filter(addedFields::add)
               .forEach(field -> expandedFields.add(context.relBuilder.field(field)));
         }
+        case Alias alias -> expandedFields.add(rexVisitor.analyze(alias, context));
         default ->
             throw new IllegalStateException(
                 "Unexpected expression type in project list: " + expr.getClass().getSimpleName());