[VL] ExpandExecTransformer should align projection types with Expand output attributes

### Backend

VL (Velox)

### Bug description

### Backend

VL (Velox)

### Bug description

`ExpandExecTransformer` currently converts Spark `ExpandExec.projections` to Substrait `ExpandRel` mostly according to each projection expression's own type. However, Spark's `ExpandExec.output` attributes are the schema contract for each Expand output column.

In some multiple-distinct aggregate plans, especially with decimal expressions, `CASE WHEN`, `round(avg(decimal))`, or duplicated grouping keys, a projection expression's Spark/Gluten transformer type can differ from the corresponding `ExpandExec.output` attribute type. This may produce an ExpandRel whose per-column projection types are inconsistent with the output schema.

Expected behavior:

Gluten should make sure every Expand projection column is type-compatible with the corresponding `ExpandExec.output` attribute before Substrait conversion. If the backend cannot support the required alignment in native Expand, validation should fail cleanly and fallback, instead of producing an invalid native plan or failing later.

Actual behavior:

Plans can hit native planning/validation failures or fallback reasons such as:

```text
Reason: The projections type does not match across different rows in the same column. Got: DECIMAL(27, 10), DECIMAL(28, 10)
Retriable: False
Expression: projections_[j][i]->type()->equivalent(*type)
Function: ExpandNode
File: /home/jenkins/agent/workspace/di-spark/Gluten/CI_spark_gluten/ep/build-velox/build/velox_ep/velox/core/PlanNode.cpp
Line: 511
Stack trace:
# 0 _ZN8facebook5velox7process10StackTraceC1Ei
# 1 _ZN8facebook5velox14VeloxExceptionC1EPKcmS3_St17basic_string_viewIcSt11char_traitsIcEES7_S7_S7_bNS1_4TypeES7_
# 2 _ZN8facebook5velox6detail14veloxCheckFailINS0_14VeloxUserErrorERKSsEEvRKNS1_18VeloxCheckFailArgsET0_
# 3 _ZN8facebook5velox4core10ExpandNodeC1ESsSt6vectorIS3_ISt10shared_ptrIKNS1_10ITypedExprEESaIS7_EESaIS9_EES3_ISsSaISsEES4_IKNS1_8PlanNodeEE
# 4 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait9ExpandRelE
# 5 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait3RelE
# 6 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait12AggregateRelE
# 7 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait3RelE
# 8 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait10ProjectRelE
# 9 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait3RelE
# 10 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait10ProjectRelE
# 11 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait3RelE
# 12 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait7RelRootE
# 13 _ZN6gluten29SubstraitToVeloxPlanConverter11toVeloxPlanERKN9substrait4PlanE
# 14 _ZN6gluten18VeloxPlanConverter11toVeloxPlanERKN9substrait4PlanESt6vectorINS1_18ReadRel_LocalFilesESaIS6_EE
# 15 _ZN6gluten12VeloxRuntime20createResultIteratorERKSsRKSt6vectorISt10shared_ptrINS_14ResultIteratorEESaIS6_EERKSt13unordered_mapISsSsSt4hashISsESt8equal_toISsESaISt4pairIS1_SsEEE
# 16 Java_org_apache_gluten_vectorized_PlanEvaluatorJniWrapper_nativeCreateKernelWithIterator
# 17 0x00007f94490186e7

at org.apache.gluten.vectorized.PlanEvaluatorJniWrapper.nativeCreateKernelWithIterator(Native Method)
at org.apache.gluten.vectorized.NativePlanEvaluator.createKernelWithBatchIterator(NativePlanEvaluator.java:69)
at org.apache.gluten.backendsapi.velox.VeloxIteratorApi.genFinalStageIterator(VeloxIteratorApi.scala:255)
```

Reproducer
```sql
CREATE TABLE pending_events (
order_id BIGINT,
pending_date DATE,
pending_reason STRING,
pending_timestamp BIGINT
) USING PARQUET;

CREATE TABLE verified_events (
order_id BIGINT,
verified_timestamp BIGINT
) USING PARQUET;

INSERT INTO pending_events VALUES
(1001, DATE '2026-04-20', 'A', 0),
(1002, DATE '2026-04-20', 'A', 0),
(1003, DATE '2026-04-20', 'B', 0),
(1004, DATE '2026-04-21', 'A', 0);

INSERT INTO verified_events VALUES
(1001, 43200), -- 12.0 hours
(1002, 108000), -- 30.0 hours
(1003, 198000), -- 55.0 hours
(1004, 176400); -- 49.0 hours

WITH sla_calc AS (
SELECT
p.pending_date,
p.pending_reason,
p.order_id,
ROUND(
CAST(v.verified_timestamp - p.pending_timestamp AS DECIMAL(38, 18))
/ CAST(3600 AS DECIMAL(38, 18)),
1
) AS sla_hours
FROM pending_events p
JOIN verified_events v
ON p.order_id = v.order_id
)

SELECT
pending_date,
pending_reason,
COUNT(DISTINCT order_id) AS total_order,
ROUND(AVG(sla_hours), 1) AS avg_sla_hours,
COUNT(DISTINCT CASE WHEN sla_hours > 24 THEN order_id END) AS backlog_24,
COUNT(DISTINCT CASE WHEN sla_hours > 48 THEN order_id END) AS backlog_48
FROM sla_calc
GROUP BY pending_date, pending_reason;

DROP TABLE IF EXISTS pending_events;
DROP TABLE IF EXISTS verified_events;
```
Possible fix direction
Before converting Expand projections to Substrait:

- Compare each projection expression type with the corresponding ExpandExec.output attribute type.
- Align null literals directly to the output type.
- For other mismatches, either insert an explicit cast in a place supported by the backend, or push the cast into a pre-project if native Expand cannot contain scalar functions.
- During validation, return ValidationResult.failed(...) for unsupported alignment cases so planning can fallback cleanly.
- Keep a hard assertion/protection in doTransform to avoid generating invalid native ExpandRel after validation.

This issue was written with the assistance of AI.

### Gluten version

Gluten-1.3

### Spark version

Spark-3.2.x

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```

Start at ExpandExecTransformer and its doTransform and validation paths, comparing each ExpandExec projection with the corresponding ExpandExec.output attribute before Substrait ExpandRel conversion. Use the supplied SQL reproducer and verify that unsupported mismatches return ValidationResult.failed(...) and fallback rather than producing an invalid native Expand plan.

Written by the indexing model from the issue text.