fix(plugin-system): load/enable failures, atomic state files, pip lock, test-double parity (#645)

- load_plugin: an on_enable() that raises unregisters the instance, so the
  next load retries instead of returning True "already loaded".
- get_plugin_info: guard plugin.get_info(); one plugin raising no longer
  breaks /api/v3/plugins/installed.
- plugin_state.json and the operation history are written with
  atomic_write_text under their lock.
- plugin_loader: module-level lock serialises pip installs across the
  parallel startup loaders.
- store_manager._install_via_download: extract dir cleanup moved to finally.
- Test doubles: draw_image() warns (DeprecationWarning; the real
  DisplayManager has none), MockDisplayManager.draw_text accepts the real
  signature's optional params, VisualTestDisplayManager logs draw errors at
  WARNING.
- Docs/comments: compatibility.py method name, PluginState.LOADED meaning,
  brittle schema count, why _report_skip_once uses setdefault.
- Remove unused PluginOperationQueue.get_active_operations().

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/plugin_system/operation_queue.py

@@ -172,20 +172,6 @@ class PluginOperationQueue:
             )
             return history[:limit]
     
-    def get_active_operations(self) -> List[PluginOperation]:
-        """
-        Get all currently active operations (pending or running).
-        
-        Returns:
-            List of active operations
-        """
-        with self._lock:
-            active = []
-            for operation in self._operations.values():
-                if operation.status in [OperationStatus.PENDING, OperationStatus.RUNNING]:
-                    active.append(operation)
-            return active
-    
     def _start_worker(self) -> None:
         """Start the worker thread that processes operations."""
         if self._worker_thread and self._worker_thread.is_alive():