feat(core): add MaskExpression POJO and ReadRel projection support

[flex-seongmin]

Summary

Implement the MaskExpression POJO layer for ReadRel projection support in substrait-java.

The Substrait protobuf spec defines Expression.MaskExpression as a field on ReadRel (field 4: projection), but the Java POJO layer had only a TODO comment and no actual implementation. This means any ReadRel projection information was silently dropped during proto-to-POJO conversion, and POJO-constructed plans could not express column projection at the read level.

This change closes that gap by introducing a full POJO model for MaskExpression and wiring it through the existing conversion infrastructure.

Changes

• MaskExpression.java (new) -- @Value.Enclosing interface with 11 @Value.Immutable inner types (MaskExpr, Select, StructSelect, StructItem, ListSelect, ListSelectItem, ListElement, ListSlice, MapSelect, MapKey, MapKeyExpression) matching the proto definition 1:1
• MaskExpressionProtoConverter.java (new) -- Dedicated bidirectional converter between io.substrait.proto.Expression.MaskExpression and the POJO types, keeping proto FQN references isolated from the relation converters
• AbstractReadRel.java -- Replaced TODO comment with Optional<MaskExpression.MaskExpr> getProjection()
• ProtoRelConverter.java -- Added optionalMaskExpression() helper; all four ReadRel subtypes (NamedScan, LocalFiles, VirtualTableScan, ExtensionTable) now populate projection during proto-to-POJO conversion
• RelProtoConverter.java -- All four visit(ReadRel) methods now serialize projection back to proto via MaskExpressionProtoConverter.toProto()

Testing

Added 11 roundtrip tests in ReadRelRoundtripTest covering:

• Simple flat column projection (StructSelect only)
• Nested struct projection (StructItem with child Select.ofStruct)
• List element and slice selection (ListSelect)
• List selection with nested child (ListSelect + child StructSelect)
• Map key selection (MapSelect with MapKey)
• Map key expression / wildcard selection (MapKeyExpression)
• Map selection with nested child (MapSelect + child StructSelect)
• maintainSingularStruct flag
• Projection combined with filter
• Projection on VirtualTableScan and LocalFiles

[CLAassistant]

All committers have signed the CLA.

[nielspardon]

Hi @flex-seongmin, thanks for your contribution. I enabled the Github Action workflow and it looks like your PR has some formatting issues. Running ./gradlew spotlessApply should automatically fix these for you.

[nielspardon]

I have some initial structural suggestions for the code from my side

[flex-seongmin]

Thanks for kind direction for sync code convention and architectural consistency.

I added 3 commits.

[flex-seongmin]

I committed that you reviewed! Thanks for review.

• Added javadoc for class / public functions
• Use MaskExpressionVisitor for Select dispatc.

[nielspardon]

if you can fix the javadoc and the minor renaming I think this is looking good

[nielspardon]

I think this is also missing a @return

[nielspardon]

Suggested change

	/** The top-level struct selection describing which fields to include. */
	/**
	* The top-level struct selection describing which fields to include.
	*
	* @return the top-level struct selection
	*/

[flex-seongmin]

I apologize that I didn't check it carefully. Adjusted It

[nielspardon]

no worries. I seem to have given you a code suggestion that isn't formatted properly by accident and you would need to run spotlessApply to fix the indendation

[flex-seongmin]

@nielspardon I executed spotlessApply task for lint

[nielspardon]

thanks

[nielspardon]

LGTM, thank you