[CH] sparkArrayFold aggregate fails with lambda type mismatch on nullable struct inputs

### Backend

CH (Clickhouse)

### Bug description

## Description
`aggregate` (mapped to sparkArrayFold) fails when lambda argument types (accumulator/element) do not exactly match the runtime-captured types, especially with nullable arrays or nullable struct fields. This leads to runtime exceptions or incorrect null results.

## Steps to Reproduce
1. Create a table with array> including null elements/fields.
2. Run aggregate with merge + finish lambda, e.g.:
- merge lambda uses struct accumulator
- finish lambda reads accumulator fields

## Expected Behavior
Results match vanilla Spark.

## Actual Behavior
Runtime error (type mismatch in lambda capture) or incorrect null outputs.

## Proposed Fix
- Align accumulator/element types with merge lambda argument types.
- Cast array elements to the expected lambda element type when needed.
- Add tests for nested struct + nulls.

## Steps to Reproduce
-- setup
```
CREATE TABLE tb_array_complex(items ARRAY>) USING parquet;

INSERT INTO tb_array_complex VALUES
(array(named_struct('v', 1, 'w', 1.5), named_struct('v', null, 'w', 2.0), null)),
(array()),
(null),
(array(named_struct('v', 2, 'w', null), named_struct('v', 3, 'w', 4.5)));
```

-- reproduce
```
SELECT
aggregate(
items,
cast(struct(0 as cnt, 0.0 as sum) as struct),
(acc, x) -> struct(
acc.cnt + if(x is null or x.v is null, 0, 1),
acc.sum + coalesce(x.w, 0.0)
),
acc -> if(acc.cnt = 0, cast(null as double), acc.sum / acc.cnt)
) AS avg_w
FROM tb_array_complex;
```

-- setup
```
CREATE TABLE tb_array_simple(ids ARRAY) USING parquet;
INSERT INTO tb_array_simple VALUES (array(1,5,2,null,3)), (array(1,1,3,2)), (null), (array());
```

-- reproduce
```
SELECT
aggregate(
ids,
cast(struct(0 as cnt, 0.0 as sum) as struct),
(acc, x) -> struct(acc.cnt + 1, acc.sum + coalesce(cast(x as double), 0.0)),
acc -> acc.sum
) AS sum_v
FROM tb_array_simple;
```

### Gluten version

main branch

### Spark version

Spark-3.3.x

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```

Start at the ClickHouse backend's aggregate implementation mapped to sparkArrayFold, then run the two supplied SQL reproductions with nullable structs, arrays, and lambda captures. Compare the results with vanilla Spark; done means matching results and coverage for nested structs and null elements.

Written by the indexing model from the issue text.