test(cli): cover recursive prefix download paths

[overtrue]

Summary

• Add one integration regression test for recursive S3 prefix downloads into a local directory.
• Cover the recent download_prefix path branch so stripped keys like /dir/a.txt stay under the requested destination.

Evidence

• Recent change bccd595 trimmed leading slashes in download_prefix after strip_prefix.
• Existing recursive copy coverage checks S3-to-S3, but did not assert S3-to-local prefix download layout.

Validation

• cargo test -p rustfs-cli --features integration --test integration test_recursive_download_prefix_keeps_paths_under_destination -- --nocapture
• make pre-commit

Note: the focused integration test used the existing skip path on this machine because S3 test credentials are not configured.

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