Add retry with exponential backoff for upload/download

[AndreyVMarkelov]

Summary

Add retry with exponential backoff for transient errors on uploads and downloads.

• Retry on server 500s, rate limits (with RetryAfter), network errors, unexpected EOF, and too_many_write_operations
• Downloads write to atomic temp file (.name.tmp-PID-NANO) then rename — prevents corruption on retry or crash
• Chunked uploads handle already-accepted chunks idempotently via incorrect_offset detection
• Symlinks are preserved on download (temp file created alongside resolved target)
• Single-shot uploads seek back to start before each retry attempt

Issues Fixed

• Closes Error post upload_session/append_v2 connection reset by peer #211 — connection reset by peer on upload_session/append_v2
• Closes Error when uploading a large file #185 — large file upload fails with HTTP 500
• Closes dbxcli download issues ( Error: unexpected EOF ) #75 — download fails with unexpected EOF on large files
• Closes Receiving Error: too_many_requests/. when trying to PUT a file #149 — too_many_requests error on put

Test Plan

• Unit tests for retry logic (transient vs permanent errors, backoff timing, rate-limit RetryAfter)
• Unit tests for atomic download (body read error retry, temp file cleanup, symlink preservation)
• Unit tests for chunk idempotency (expected vs unexpected incorrect_offset)
• Unit tests for too_many_write_operations retry (single-shot and session finish)
• Manual test: upload + download + verify file integrity (including zip files)
• go vet / go build clean

[CLAassistant]

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