[BUG] Calcite PPL doesn't handle array value columns if codegen triggered

What is the bug?
If there is codegen triggered, i.e. not fully pushed down into DSL, PPL will fail when performing any array operation on an array value field.

How can one reproduce the bug?
Steps to reproduce the behavior:

###
POST localhost:9200/test_idx/_doc
Content-Type: application/json

{ "nums": [1, 2, 3] }

###
POST localhost:9200/_plugins/_ppl
Content-Type: application/json

{
  "query": "source = test_idx | expand nums"
}

Actual Result:

{
  "error": {
    "reason": "There was internal problem at backend",
    "details": "UNNEST argument must be a collection",
    "type": "CalciteException"
  },
  "status": 500
}

Exception and stack-trace in cluster:

[2026-01-23T13:16:27,185][ERROR][o.o.s.p.r.RestPPLQueryAction] [integTest-0] Error happened during query handling
org.apache.calcite.runtime.CalciteException: UNNEST argument must be a collection
        at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62)
        at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502)
        at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:486)
        at org.apache.calcite.runtime.Resources$ExInstWithCause.ex(Resources.java:511)
        at org.apache.calcite.runtime.Resources$ExInst.ex(Resources.java:605)
        at org.apache.calcite.rel.core.Uncollect.deriveUncollectRowType(Uncollect.java:177)
        at org.apache.calcite.rel.core.Uncollect.deriveRowType(Uncollect.java:131)
        at org.apache.calcite.rel.core.Uncollect.<init>(Uncollect.java:82)
        at org.apache.calcite.tools.RelBuilder.uncollect(RelBuilder.java:2368)
        at org.opensearch.sql.calcite.CalciteRelNodeVisitor.buildExpandRelNode(CalciteRelNodeVisitor.java:3354)
        at org.opensearch.sql.calcite.CalciteRelNodeVisitor.visitExpand(CalciteRelNodeVisitor.java:3084)
        at org.opensearch.sql.calcite.CalciteRelNodeVisitor.visitExpand(CalciteRelNodeVisitor.java:168)
        at org.opensearch.sql.ast.tree.Expand.accept(Expand.java:41)
        at org.opensearch.sql.ast.AbstractNodeVisitor.visitChildren(AbstractNodeVisitor.java:111)
        at org.opensearch.sql.calcite.CalciteRelNodeVisitor.visitProject(CalciteRelNodeVisitor.java:370)
        at org.opensearch.sql.calcite.CalciteRelNodeVisitor.visitProject(CalciteRelNodeVisitor.java:168)
        at org.opensearch.sql.ast.tree.Project.accept(Project.java:65)
        at org.opensearch.sql.calcite.CalciteRelNodeVisitor.analyze(CalciteRelNodeVisitor.java:181)
        at org.opensearch.sql.executor.QueryService.analyze(QueryService.java:256)
        at org.opensearch.sql.executor.QueryService.lambda$executeWithCalcite$0(QueryService.java:106)
        at org.opensearch.sql.calcite.CalcitePlanContext.run(CalcitePlanContext.java:161)
        at org.opensearch.sql.executor.QueryService.executeWithCalcite(QueryService.java:96)
        at org.opensearch.sql.executor.QueryService.execute(QueryService.java:73)
        at org.opensearch.sql.executor.execution.QueryPlan.execute(QueryPlan.java:66)
        at org.opensearch.sql.opensearch.executor.OpenSearchQueryManager.lambda$schedule$1(OpenSearchQueryManager.java:64)
        at org.opensearch.sql.opensearch.executor.OpenSearchQueryManager.lambda$withCurrentContext$2(OpenSearchQueryManager.java:89)
        at org.opensearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:916)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
        at java.base/java.lang.Thread.run(Thread.java:1583)

Root Cause Analysis

OpenSearch index doesn't has type of ARRAY, and the dynamic mapping parses the field nums to be type of long. So our type system translate that into BasicSqlType(SqlTypeName.BIGINT) in Calcite while Calcite's Uncollect operator only accepts ArraySqlType

What is the expected behavior?
The query should succeed with nums exploded.

What is your host/environment?

Do you have any screenshots?
If applicable, add screenshots to help explain your problem.

Do you have any additional context?
Related issues: https://github.com/opensearch-project/sql/issues/4173, it has fixed by https://github.com/opensearch-project/sql/pull/5015 while that PR cannot handle the above case.

Reproduce the failure with the provided index document and PPL expand query, then start at CalciteRelNodeVisitor.buildExpandRelNode and trace how the nums field type is translated before Calcite's Uncollect operator. Compare the related issue 4173 and pull request 5015, and verify that the query succeeds with nums exploded without breaking existing array operations.