fix(manager): drop the distinfo entry when a plugin is unregistered

[ciperx]

Fixes #763.

PluginManager.unregister() removed the plugin from its hook callers and from _name2plugin, but left its _plugin_distinfo entry in place. For a plugin loaded through entry points that means:

• list_plugin_distinfo() and list_plugin_distributions() kept reporting the plugin after it had been unregistered, so the listings disagreed with get_plugin() / has_plugin().
• Re-registering the plugin appended a second entry for the same plugin, so the listings grew on every unregister/reload cycle.

Change

unregister() now also removes the distinfo entries belonging to that plugin:

self._plugin_distinfo[:] = [
    (p, dist) for p, dist in self._plugin_distinfo if p is not plugin
]

Rebinding in place (rather than assigning a new list) keeps the existing list object, since the attribute is annotated Final. No public API change, no new dependency.

Regression coverage

testing/test_unregister_distinfo.py covers three paths:

• unregistering by name removes the plugin from both list_plugin_distinfo() and list_plugin_distributions();
• unregister then register does not leave a stale entry, and a later entry-point reload produces exactly one entry again;
• unregistering by plugin object clears the bookkeeping as well.

Verification

$ python -m pytest testing/ -q
222 passed

With the _plugin_distinfo cleanup reverted, all three new tests fail (3 failed), so the coverage is not vacuous.

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This work was completed for OpenJobs listing eb2edb80-4a8f-4a90-9d26-859c4b977f65 (https://openjobs.bot/jobs/eb2edb80-4a8f-4a90-9d26-859c4b977f65).