Add aliases for session handshake part conversions

[oferchen]

Summary

• add SessionHandshake::into_parts and SessionHandshake::from_parts so the aggregated session helper mirrors the variant-specific API shape
• keep the legacy into_stream_parts/from_stream_parts entry points as thin aliases to maintain backwards compatibility
• add focused tests ensuring the new aliases behave identically for binary and legacy negotiations

Testing

• cargo test -q -p rsync-transport session_handshake_into_parts_aliases_method_for_binary
• cargo test -q -p rsync-transport session_handshake_into_parts_aliases_method_for_legacy

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https://chatgpt.com/codex/tasks/task_e_68fbb45ae9a0832394a8c7a89f4201b1