[Fix](ctas) forward the use_max_length_of_varchar_in_ctas (#39705) (#40138)

cherry-pick from master #39705

fe/fe-core/src/main/java/org/apache/doris/qe/SessionVariable.java

@@ -1996,7 +1996,7 @@ public class SessionVariable implements Serializable, Writable {
             checker = "checkExternalAggPartitionBits", fuzzy = true)
     public int externalAggPartitionBits = 5; // means that the hash table will be partitioned into 32 blocks.
 
-    @VariableMgr.VarAttr(name = USE_MAX_LENGTH_OF_VARCHAR_IN_CTAS, description = {
+    @VariableMgr.VarAttr(name = USE_MAX_LENGTH_OF_VARCHAR_IN_CTAS, needForward = true, description = {
             "在CTAS中，如果 CHAR / VARCHAR 列不来自于源表，是否是将这一列的长度设置为 MAX，即65533。默认为 true。",
             "In CTAS (Create Table As Select), if CHAR/VARCHAR columns do not originate from the source table,"
                     + " whether to set the length of such a column to MAX, which is 65533. The default is true."