core: Rel.Remap validates none of its indices, so a malformed emit mapping fails as a bare index exception deep in a consumer

[nielspardon]

Rel.Remap holds nothing but its index list and validates none of it.

substrait-java/core/src/main/java/io/substrait/relation/Rel.java

Lines 152 to 183 in 934a60e

	/**
	* Reorders and/or selects a relation's output fields by index, producing the emitted record type.
	*/
	@Value.Immutable
	abstract class Remap {
	/**
	* Returns the output field indices, in emission order.
	*
	* @return the field indices
	*/
	public abstract List<Integer> indices();
	/**
	* Applies this remap to the given record type, selecting and reordering fields by index.
	*
	* @param initial the record type to remap
	* @return the remapped record type
	*/
	public Type.Struct remap(Type.Struct initial) {
	List<Type> types = initial.fields();
	return TypeCreator.of(initial.nullable()).struct(indices().stream().map(i -> types.get(i)));
	}
	/**
	* Creates a remap that emits the given field indices in order.
	*
	* @param fields the field indices to emit
	* @return the remap
	*/
	public static Remap of(Iterable<Integer> fields) {
	return ImmutableRemap.builder().addAllIndices(fields).build();
	}

ProtoRelConverter copies common.emit.output_mapping through verbatim, so whatever a producer wrote reaches the POJO, and the first thing to fail is whoever indexes with it.

An out-of-range index reaches remap(Type.Struct)'s types.get(i), or isthmus's applyRemap:

substrait-java/isthmus/src/main/java/io/substrait/isthmus/SubstraitRelNodeConverter.java

Lines 1310 to 1319 in 934a60e

	private RelNode applyRemap(RelNode relNode, Rel.Remap remap) {
	RelDataType rowType = relNode.getRowType();
	// By index rather than by name: a virtual table's row type comes straight from its schema,
	// which Calcite never uniquifies, so a name can stand for more than one field.
	List<RexNode> rexList =
	remap.indices().stream()
	.map(index -> new RexInputRef(index, rowType.getFieldList().get(index).getType()))
	.collect(java.util.stream.Collectors.toList());
	return relBuilder.push(relNode).project(rexList).build();
	}

For a two-column relation and emit = [5] that is java.lang.IndexOutOfBoundsException: Index 5 out of bounds for length 2, naming neither the relation nor the mapping. An empty index list builds too, and yields a node with no columns that no Substrait relation describes.

remap(Type.Struct) is where the arity is known, so a range check belongs there, naming the index and the arity as an IllegalArgumentException. Whether a repeated index is legal is a spec question rather than one to settle here: it currently converts, and isthmus silently invents a name for the second copy (Remap.of(List.of(0, 0)) over [col1, col2] comes back as [col1, col10] through Calcite's uniquifier).

Found while reviewing #1189.