[Bug](agg-state) fix agg state inner type not conversioned (#50418)

### What problem does this PR solve?


drop table if exists t01;
create table t01(id int, decimal_col agg_state<sum(decimal(20,6))>
generic) properties ("replication_num" = "1");
insert into t01 values (1, sum_state(10.1)), (1, sum_state(20.1)), (2,
sum_state(10.2)), (2, sum_state(11.0));
select sum_merge(decimal_col) from t01 group by id;
 

[18:36]MySQL [email protected]:test_agg_state> select
sum_merge(decimal_col) from t01 group by id; (1105, "errCode = 2,
detailMessage =
(172.20.48.119)[INTERNAL_ERROR]AggState(function_name=sum,result_is_nullable=true,arguments=[Nullable(Decimal(27,
9))],be_exec_version=8)'s expect return type is Nullable(Decimal(27,
9)), but input Nullable(Decimal(76, 6))") [18:36]MySQL
[email protected]:test_agg_state>


### Check List (For Author)

- Test <!-- At least one of them must be included. -->
    - [x] Regression test
    - [ ] Unit Test
    - [ ] Manual test (add detailed scripts or steps below)
    - [ ] No need to test or manual test. Explain why:
- [ ] This is a refactor/code format and no logic has been changed.
        - [ ] Previous test can cover this change.
        - [ ] No code files have been changed.
        - [ ] Other reason <!-- Add your reason?  -->

- Behavior changed:
    - [x] No.
    - [ ] Yes. <!-- Explain the behavior change -->

- Does this need documentation?
    - [x] No.
- [ ] Yes. <!-- Add document PR link here. eg:
apache/doris-website#1214 -->

### Check List (For Reviewer who merge this PR)

- [ ] Confirm the release note
- [ ] Confirm test cases
- [ ] Confirm document
- [ ] Add branch pick label <!-- Add branch pick label that this PR
should merge into -->

Author: BiteTheDDDDt

fe/fe-core/src/main/java/org/apache/doris/nereids/types/AggStateType.java

@@ -104,6 +104,12 @@ public Type toCatalogDataType() {
         return Expr.createAggStateType(functionName, types, subTypeNullables);
     }
 
+    @Override
+    public DataType conversion() {
+        return new AggStateType(functionName, subTypes.stream().map(DataType::conversion).collect(Collectors.toList()),
+                subTypeNullables);
+    }
+
     @Override
     public boolean acceptsType(DataType other) {
         return other instanceof AggStateType;

regression-test/data/datatype_p0/agg_state/decimal_sum/test_decimal_sum.out

@@ -0,0 +1,5 @@
+-- This file is automatically generated. You should know what you did if you want to edit this
+-- !select --
+30.200000
+21.200000
+

regression-test/suites/datatype_p0/agg_state/decimal_sum/test_decimal_sum.groovy

@@ -0,0 +1,31 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+suite("test_decimal_sum") {
+    sql "set enable_agg_state=true"
+    sql """ DROP TABLE IF EXISTS t01; """
+    sql """
+        create table t01(id int, decimal_col agg_state<sum(decimal(20,6))> generic)  properties ("replication_num" = "1");
+        """
+
+    sql """insert into t01 values (1, sum_state(10.1)), (1, sum_state(20.1)), (2, sum_state(10.2)), (2, sum_state(11.0));
+"""
+
+
+    qt_select """ select sum_merge(decimal_col) from t01 group by id order by id;
+             """
+}