Add plugin enhancements: PluginStatus, PluginError, subscribe_plugin, config validation

- New: fmfuncs/plugin_status.py (9 states)
- New: fmfuncs/plugin_error.py (6 exception classes)
- Enhanced: bridges/plugin_bridge.py (subscribe_plugin)
- Enhanced: bridges/plugin_network_bridge.py (setup_data_transfer, send_data)
- Enhanced: services/plugin_service.py (config validation + status tracking)
- Tests: 15/15 passing (10 original + 5 new)

fmfuncs/plugin_status.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""插件状态枚举 — 统一的插件生命周期状态跟踪"""
+from enum import Enum
+
+class PluginStatus(str, Enum):
+    UNLOADED = "unloaded"
+    LOADING = "loading"
+    LOADED = "loaded"
+    INITIALIZING = "initializing"
+    RUNNING = "running"
+    ERROR = "error"
+    STOPPING = "stopping"
+    STOPPED = "stopped"
+    UNLOADING = "unloading"