Union all pushdown unsupported scalar function CastJsonAsJson

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

use test;
drop table if exists t;
drop table if exists t1;
create table t (a json not null, b json, c varchar(20));
create table t1 (a json, b json not null, c double);
alter table t set tiflash replica 1;
alter table t1 set tiflash replica 1;
set tidb_enforce_mpp=1; set tidb_isolation_read_engines='tiflash';
select count(*) from (select a , b from t union all select a , b from t1) tt;

2. What did you expect to see? (Required)

3. What did you see instead (Required)

ERROR 1105 (HY000): other error for mpp stream: DB::TiFlashException: CastJsonAsJson is not supported.

[tidb]> desc select count(*) from (select a , b from t union all select a , b from t1) tt;
+------------------------------------+----------+--------------+---------------+----------------------------------------------------+
| id                                 | estRows  | task         | access object | operator info                                      |
+------------------------------------+----------+--------------+---------------+----------------------------------------------------+
| HashAgg_37                         | 1.00     | root         |               | funcs:count(Column#12)->Column#11                  |
| └─TableReader_39                   | 1.00     | root         |               | data:ExchangeSender_38                             |
|   └─ExchangeSender_38              | 1.00     | cop[tiflash] |               | ExchangeType: PassThrough                          |
|     └─HashAgg_14                   | 1.00     | cop[tiflash] |               | funcs:count(1)->Column#12                          |
|       └─Union_36                   | 20000.00 | cop[tiflash] |               |                                                    |
|         ├─Projection_64            | 10000.00 | cop[tiflash] |               | cast(test.t.a, json BINARY)->Column#9, test.t.b    |
|         │ └─TableFullScan_22       | 10000.00 | cop[tiflash] | table:t       | keep order:false, stats:pseudo                     |
|         └─Projection_65            | 10000.00 | cop[tiflash] |               | test.t1.a, cast(test.t1.b, json BINARY)->Column#10 |
|           └─TableFullScan_29       | 10000.00 | cop[tiflash] | table:t1      | keep order:false, stats:pseudo                     |
+------------------------------------+----------+--------------+---------------+----------------------------------------------------+
9 rows in set (0.001 sec)

4. What is your TiFlash version? (Required)

master

Start with the supplied SQL reproduction and its DESC output, focusing on the Union All projections that produce CastJsonAsJson under TiFlash MPP. Trace where this scalar function is handled for pushdown, then verify that the query completes without the unsupported-function error.