Large test bundle uploads no longer OOM the build job

Uploads of large test bundles (multi-GB, hundreds of modules) now keep at most 10 multipart parts in flight at once, so peak memory per upload is bounded by a fixed multiple of the part size instead of growing with the artifact. Each module’s .xctest is also archived in place from the products directory rather than copied to a staging directory first, avoiding tens of GB of redundant disk I/O per build on very large projects. Together these fix the upload OOMs and excessive disk usage that large selective-testing runs hit before.