feat: Increase file size limit from 25GB to 50.1GB

[sbassam]

Have you read the Contributing Guidelines?

Issue #

Describe your changes

Clearly and concisely describe what's in this pull request. Include screenshots, if necessary.

---

Note

Increases max file size to 50.1GB, switches multipart target part size to 250MB with sliding-window concurrency, adds progress bars to validation, and sets a download request timeout.

• Uploads:
  • Multipart: Set TARGET_PART_SIZE_MB to 250; keep MAX_MULTIPART_PARTS at 250.
  • Implement sliding-window concurrency in MultipartUploadManager._upload_parts_concurrent to respect max_concurrent_parts while continuously feeding new parts.
  • Increase max supported file size to MAX_FILE_SIZE_GB = 50.1.
• Download:
  • Add request_timeout=3600 to raw GET in DownloadManager.download.
• File validation:
  • Wrap JSONL iteration with tqdm for progress; import tqdm.
  • Improve UTF-8 check to read in chunks.
• Tests:
  • Update expectations for part sizing/count (e.g., 500MB → 2×250MB, 50GB → ~205 parts) and size-limit error message.
  • Adjust as_completed mocking to align with sliding-window behavior.

^{Written by Cursor Bugbot for commit 7ac3d44. This will update automatically on new commits. Configure here.}