Use reliable SDK downloads

[AndreyVMarkelov]

Summary

• Adopt the Go SDK’s filetransfer downloader for normal dbxcli get file downloads.
• This fixes silent truncated downloads such as #214, where a large downloaded zip could be incomplete but still appear successful.

Changes

• Use resumable ranged downloads with transient-failure retries.
• Validate downloaded size and Dropbox content_hash before replacing the destination.
• Preserve atomic destination writes and symlink resolution.
• Keep export-only files on the existing export path.
• Map SDK progress events to the current terminal progress output.
• Update the body-read regression test to verify resume via Range.
• Extend the opt-in E2E script with byte-for-byte disk and stdout download checks.
• Update CI workflows to Go 1.25.13.