uploaders/pulp: parallel chunks, direct artifacts

[andrewlukoshko]

• Replace sequential filesplit-based disk splitting with parallel ThreadPoolExecutor chunk uploads using byte-range reads via temp files
• Add direct artifact creation via ArtifactsApi.create() for files smaller than chunk_size, bypassing the 3-step upload+commit flow
• Eliminate redundant SHA256 hashing: file is now hashed once in upload_single_file and the hash is passed through _send_file and _commit_upload (was hashed 3 times per upload)
• Add per-file upload timing logs at INFO level for all three upload paths (direct artifact, single chunk, multi-chunk)
• Remove filesplit dependency from requirements.txt
• Add 5 new tests: direct artifact creation, parallel chunk upload, chunk error propagation, and timing log verification for both paths