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/plugin_state.py

@@ -17,7 +17,7 @@ from src.logging_config import get_logger
 class PluginState(Enum):
     """Plugin state enumeration."""
     UNLOADED = "unloaded"  # Plugin not loaded
-    LOADED = "loaded"  # Plugin module loaded but not instantiated
+    LOADED = "loaded"  # load_plugin() in progress: set before the module is imported
     ENABLED = "enabled"  # Plugin instantiated and enabled
     RUNNING = "running"  # Plugin is currently executing
     ERROR = "error"  # Plugin encountered an error