Add resilient file transfer helpers

[AndreyVMarkelov]

Summary

• add resilient file-transfer helpers for sequential, concurrent, and reader-backed uploads
• reconcile upload-session offsets after ambiguous responses and validate source ranges
• honor Retry-After for local file-download retries
• add unit, type, and authenticated integration coverage

[codecov]

Codecov Report

❌ Patch coverage is 84.69388% with 45 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.38%. Comparing base (d67ed8a) to head (5439aad).

Files with missing lines	Patch %	Lines
src/filetransfer.js	83.92%	45 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1205      +/-   ##
==========================================
+ Coverage   72.89%   75.38%   +2.49%     
==========================================
  Files          10       11       +1     
  Lines        1199     1491     +292     
==========================================
+ Hits          874     1124     +250     
- Misses        325      367      +42     

Flag	Coverage Δ
integration	49.02% <52.04%> (+0.90%)	⬆️
unit	74.58% <84.69%> (+2.43%)	⬆️

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