JDBC: Fix stackoverflow on getObject and timestamp conversion (#31735)

StackOverflowError fix in JdbcResultSet getObject method.
Fix Timestamp conversion bug when getting the value of a time column.

x-pack/plugin/sql/jdbc/src/main/java/org/elasticsearch/xpack/sql/jdbc/jdbc/JdbcResultSet.java

@@ -344,7 +344,7 @@ public <T> T getObject(int columnIndex, Class<T> type) throws SQLException {
             throw new SQLException("type is null");
         }
 
-        return getObject(columnIndex, type);
+        return convert(columnIndex, type);
     }
 
     private <T> T convert(int columnIndex, Class<T> type) throws SQLException {

x-pack/plugin/sql/jdbc/src/main/java/org/elasticsearch/xpack/sql/jdbc/jdbc/TypeConverter.java

@@ -206,7 +206,7 @@ static Object convert(Object v, JDBCType columnType) throws SQLException {
             case REAL:
                 return floatValue(v);  // Float might be represented as string for infinity and NaN values
             case TIMESTAMP:
-                return ((Number) v).longValue();
+                return new Timestamp(((Number) v).longValue());
             default:
                 throw new SQLException("Unexpected column type [" + columnType.getName() + "]");
 

x-pack/plugin/sql/jdbc/src/test/java/org/elasticsearch/xpack/sql/jdbc/jdbc/TypeConverterTests.java

@@ -15,6 +15,7 @@
 import org.joda.time.DateTime;
 
 import java.sql.JDBCType;
+import java.sql.Timestamp;
 
 import static org.hamcrest.Matchers.instanceOf;
 
@@ -42,8 +43,8 @@ public void testDoubleAsNative() throws Exception {
 
     public void testTimestampAsNative() throws Exception {
         DateTime now = DateTime.now();
-        assertThat(convertAsNative(now, JDBCType.TIMESTAMP), instanceOf(Long.class));
-        assertEquals(now.getMillis(), convertAsNative(now, JDBCType.TIMESTAMP));
+        assertThat(convertAsNative(now, JDBCType.TIMESTAMP), instanceOf(Timestamp.class));
+        assertEquals(now.getMillis(), ((Timestamp) convertAsNative(now, JDBCType.TIMESTAMP)).getTime());
     }
 
     private Object convertAsNative(Object value, JDBCType type) throws Exception {