PluginManager.register leaves partial state after validation failure

[sb123sb123]

A failed PluginManager.register() call can leave a plugin partially registered when a later hook implementation fails validation. On main at 9836e54, a plugin with a valid a_hook and invalid b_hook raises PluginValidationError but remains in the manager and leaves a_hook installed. Expected: registration failure leaves no plugin mapping or hook implementations, so callers can safely fix and retry. The register implementation currently contains a TODO noting that state should not change on failure.

[adityabagla7]

I opened a PR for this: #740

register() now validates hook implementations before the plugin is added, and drops any hook callers created only for that attempt if validation fails. The new test covers a plugin that has one valid impl, one invalid impl, and a new hook name: after PluginValidationError the manager has none of them, and a corrected plugin can be registered.

Could a maintainer review and assign the issue if that is how you track it? Thanks.