1. SetPluginEnabled(true): after DeactivatePlugin, also call Disable(name)
to rollback in-memory Enabled state (not just config).
2. on_init failure is now fatal for ActivatePlugin — returns error
and rolls back scheduler + VM (was incorrectly non-fatal).
3. TestSetPluginEnabled_BrokenPlugin_Rollback: end-to-end test with
broken plugin (invalid interval), verifies error + not Active +
not Enabled + not in config.
mirivlad