test(cors): reject object paths for bucket cors

[overtrue]

Summary

This PR closes a small test gap in the new bucket CORS path handling. The CORS commands are documented and implemented as bucket-level operations, so object-style targets such as local/my-bucket/object.txt should be rejected before any network operation is attempted.

The previous parser only split the path into alias and the remaining string, then trimmed trailing slashes. That meant local/my-bucket/object.txt was accepted as a bucket name containing a slash, which could lead users toward confusing downstream S3 errors instead of a direct usage error.

The fix keeps trailing slash support for local/my-bucket/ but rejects bucket targets that still contain a slash after normalization. The focused unit test covers this rejected object-path case alongside the existing invalid CORS bucket path cases.

Validation

• cargo test -p rustfs-cli commands::cors::tests::test_parse_bucket_path_error
• make pre-commit (unavailable: no pre-commit target exists)
• cargo fmt --all -- --check
• cargo clippy --workspace -- -D warnings
• cargo test --workspace