Keep the expanded column addressable in mvexpand

[mch2]

mvexpand over an array whose elements are records lost the column it expanded. Uncollect explodes a record element into one column per record field, so the row type becomes the element's field names and the field(arrayFieldName) lookup right after it cannot find the array's own name: mvexpand EVENTS over ARRAY<ROW<NAME, TIME>> fails with "field [EVENTS] not found; input fields are: [ID, TAGS, NAME, TIME]".

The exploded columns are re-wrapped into a single ROW column named after the array, so the expanded column keeps its name and a leaf stays addressable as <array>.<leaf> downstream. The projection is forced, because it is an identity on field references and RelBuilder would otherwise elide it and drop the rewrap.

An element that is not a record needs nothing: ARRAY, and the ARRAY a nested mapping declares, already yield exactly one column carrying the array's name. So this only adds a case, leaving every shape that works today on its existing path.

Tests pin both halves against a table with an ARRAY column and an ARRAY column: the record array gains the ROW($0, $1) rewrap above Uncollect and fails without it, and the scalar array plans exactly as before, with no rewrap.

Description

[Describe what this change achieves]

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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[github-actions]

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[github-actions]

PR Code Suggestions ✨

Explore these optional code suggestions:

Category	Suggestion	Impact
Possible issue	Add null check for array type Add a null check for arrayFieldRex.getType() before calling getComponentType(). If the array field type is null, this could result in a NullPointerException. Consider handling this edge case explicitly. core/src/main/java/org/opensearch/sql/calcite/CalciteRelNodeVisitor.java [5131-5132] -RelDataType elementType = arrayFieldRex.getType().getComponentType(); -if (elementType != null && elementType.isStruct()) { +RelDataType arrayType = arrayFieldRex.getType(); +if (arrayType != null) { + RelDataType elementType = arrayType.getComponentType(); + if (elementType != null && elementType.isStruct()) { Suggestion importance[1-10]: 3 __ Why: While adding a null check for arrayFieldRex.getType() is a defensive programming practice, the suggestion assumes an edge case that may not occur in the actual context. The arrayFieldRex is derived from correlArrayFieldAccess which is constructed earlier in the method, and its type should be guaranteed to be non-null by the Calcite framework. The suggestion adds defensive code but may not address a realistic issue in this context.	Low

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.22%. Comparing base (3f7048b) to head (16e27ba).

Files with missing lines	Patch %	Lines
.../opensearch/sql/calcite/CalciteRelNodeVisitor.java	0.00%	6 Missing ⚠️

❌ Your project check has failed because the head coverage (63.22%) is below the target coverage (99.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5810      +/-   ##
============================================
- Coverage     63.23%   63.22%   -0.01%     
  Complexity     8823     8823              
============================================
  Files           938      938              
  Lines         40236    40242       +6     
  Branches       4537     4538       +1     
============================================
  Hits          25445    25445              
- Misses        13966    13972       +6     
  Partials        825      825              

Flag	Coverage Δ
sql-engine	63.22% <0.00%> (-0.01%)	⬇️

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