spark: ToSubstraitRel.toProtoSubstrait emits a bare-relation PlanRel that ProtoPlanConverter cannot read #1418

ToSubstraitRel.toProtoSubstrait (spark/src/main/scala/io/substrait/spark/logical/ToSubstraitRel.scala) builds the proto.Plan by hand with RelProtoConverter instead of going through PlanProtoConverter. The resulting plan has a PlanRel holding a bare rel rather than a root, so it carries no output names, and it sets neither version nor execution_behavior.

substrait-java's own reader cannot load it: ProtoPlanConverter only models root PlanRels and rejects the bare relation (#1297 tracks the reader side).

Building the bytes from the POJO plan that ToSubstraitRel.convert already produces would fix all three, at the cost of emitting root instead of rel:

def toProtoSubstrait(p: LogicalPlan): Array[Byte] =
  new PlanProtoConverter().toProto(convert(p)).toByteArray

That changes the output shape of a public method, so it is a behavior change for callers that read the rel arm.

Measured on main at 1948a86 by reading the code; raised in review of #1415, which routes the method's PlanRel through NestingDetacher but leaves its shape as is.