`named_struct` with duplicate field names falls back to Spark

[andygrove]

Part of #5572.

named_struct with duplicate field names falls the whole projection back to Spark:

if (expr.names.length != expr.names.distinct.length) {
  Unsupported(Some(duplicateNamesReason))
}

spark/src/main/scala/org/apache/comet/serde/structs.scala:42-43

docs/source/user-guide/latest/expressions.md records it as "Duplicate field names fall back".

Spark permits duplicate field names in a struct and CreateNamedStruct.doGenCode builds the row positionally, so the dispatcher reproduces it exactly. StructType is supported by CometBatchKernelCodegen.isSupportedDataType recursively, and duplicate names are not a problem on the Arrow side — the kernel's output field construction is positional too. Worth confirming with a test that the resulting Arrow schema round-trips, since duplicate field names in an Arrow struct are legal but unusual.

Fix: mix CodegenDispatchFallback into CometCreateNamedStruct.

CometCreateNamedStruct also declines inside convert at :66 when a child fails to serialize, which the dispatcher cannot see; see the prerequisite issue on convert-side declines.

[RRXXZZYY]

take