Remove BufReader wrapper when copying spill files to final shuffle output

[andygrove]

Description

In MultiPartitionShuffleRepartitioner::shuffle_write(), spill files are copied to the final shuffle output using BufReader:

let mut spill_file = BufReader::new(File::open(spill_path)?);
std::io::copy(&mut spill_file, &mut output_data)?;

The BufReader wrapper is counterproductive here because:

1. std::io::copy already uses an internal buffer for the copy
2. On Linux, std::io::copy with raw File-to-File can use copy_file_range / sendfile for kernel zero-copy, but wrapping in BufReader defeats this specialization since the source is no longer a File

Proposed Change

Remove the BufReader wrapper and pass the raw File handle directly:

let mut spill_file = File::open(spill_path)?;
std::io::copy(&mut spill_file, &mut output_data)?;

This is a one-line change in native/shuffle/src/partitioners/multi_partition.rs (in the shuffle_write method).

Note: the same optimization was already applied to the new ImmediateShufflePartitioner.