unregister() argument validation relies on assert #762

🤖 Written by Claude Opus 5.5 via Claude Code for the pluggy maintainers; I prompted it, it did the work, I read it.

unregister() checks "one of name or plugin" and "plugin is not registered" with assert. Under python -O, pm.unregister(unregistered_plugin) silently returns the plugin, and pm.unregister() returns None.

Proposal: raise TypeError when neither argument is given, and ValueError when the plugin is not registered. Code that currently catches AssertionError would need to change.

Found while working on #759.