register_op silently overwrites existing ops

[github-actions]

File: leanpass/plugin_registry.py

In leanpass/plugin_registry.py the register_op function checks if name in _REGISTRY: and then does pass, allowing the subsequent assignment to overwrite the existing entry without warning. This can lead to hard‑to‑debug bugs when a plugin unintentionally replaces a built‑in operation. The function should raise an exception or at least log a warning when a duplicate registration is attempted.

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