fix(snapshots): handle completed part-file resumes

[tallyhuhu]

Summary

Handle the resume edge case where an arc-snapshots .part file is already fully downloaded, but the process stopped before it was renamed to the final archive path.

If the next request sends a range starting at the local .part length, servers can reply with 416 Range Not Satisfiable and Content-Range: bytes */N. When N matches the local .part size, treat the part file as complete and continue with the normal rename/extract flow.

Testing

• cargo fmt -p arc-snapshots --check
• env NO_PROXY=127.0.0.1,localhost no_proxy=127.0.0.1,localhost cargo test -p arc-snapshots
• cargo clippy -p arc-snapshots --all-targets -- -D warnings