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/store_manager.py

@@ -1994,6 +1994,7 @@ class PluginStoreManager:
                     tmp_file.write(chunk)
                 tmp_zip_path = tmp_file.name
             
+            temp_extract = None
             try:
                 # Extract zip
                 with zipfile.ZipFile(tmp_zip_path, 'r') as zip_ref:
@@ -2015,7 +2016,6 @@ class PluginStoreManager:
                                 f"Zip-slip detected: member {member!r} resolves outside "
                                 f"temp directory, aborting"
                             )
-                            shutil.rmtree(temp_extract, ignore_errors=True)
                             return False
                     zip_ref.extractall(temp_extract)
                     
@@ -2027,10 +2027,6 @@ class PluginStoreManager:
                     else:
                         # No root dir, move everything
                         shutil.move(str(temp_extract), str(target_path))
-                    
-                    # Cleanup temp extract dir
-                    if temp_extract.exists():
-                        shutil.rmtree(temp_extract, ignore_errors=True)
                 
                 return True
                 
@@ -2038,6 +2034,12 @@ class PluginStoreManager:
                 # Remove temporary zip file
                 if os.path.exists(tmp_zip_path):
                     os.remove(tmp_zip_path)
+                # Cleanup temp extract dir here rather than on the success
+                # path, so a failed extract or move doesn't leave a copy of
+                # the plugin in /tmp on every attempt. (Gone already when the
+                # whole dir was moved into place.)
+                if temp_extract is not None and temp_extract.exists():
+                    shutil.rmtree(temp_extract, ignore_errors=True)
             
         except Exception as e:
             self.logger.error(f"Download failed: {e}")