fix: windows unsubscribed loaded plugin not allowed (#15751)

# Description

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  > * What issue does this PR address or fix?
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On windows, after installing and loading a plugin, if you try to
unsubscribe from a loaded plugin, and on refresh, it will be an orphaned
plugin.

# Screenshots/Recordings/Graphs

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On unsubscribing from a loaded cloud plugin
<img width="1418" height="862" alt="image"
src="https://github.com/user-attachments/assets/9ba8353c-1645-4b21-86ff-2ed235f4d259"
/>

On plugin refresh
<img width="1418" height="862" alt="image"
src="https://github.com/user-attachments/assets/fd58c144-7970-4448-ba3b-e72e52980467"
/>

## Tests

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src/slic3r/plugin/PluginManager.cpp

@@ -2042,6 +2042,8 @@ bool PluginManager::delete_and_unsubscribe_cloud_plugin(const std::string& plugi
         return false;
     }
 
+    unload_plugin(plugin_key);
+
     if (!m_cloud_service.request_cloud_unsubscribe(descriptor, error)) {
         set_plugin_error(plugin_key, error);
         return false;