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)

tests/test_plugin_enhancements.py

@@ -0,0 +1,40 @@
+"""Plugin system enhancement tests"""
+import pytest, asyncio, os, sys
+sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))
+
+from fmfuncs.plugin_status import PluginStatus
+from fmfuncs.plugin_error import (
+    PluginError, PluginLoadError, PluginConfigError,
+    PluginPermissionError, PluginCommandError
+)
+
+class TestPluginStatus:
+    def test_all_statuses(self):
+        assert PluginStatus.UNLOADED == "unloaded"
+        assert PluginStatus.RUNNING == "running"
+        assert PluginStatus.ERROR == "error"
+        assert len(list(PluginStatus)) >= 8
+
+class TestPluginError:
+    def test_base_error(self):
+        e = PluginError("test message", "test_plugin")
+        assert "test_plugin" in str(e)
+        assert "test message" in str(e)
+
+    def test_error_without_plugin_name(self):
+        e = PluginError("generic error")
+        assert "unknown" in str(e)
+
+    def test_subclass_errors(self):
+        for cls in [PluginLoadError, PluginConfigError, PluginPermissionError, PluginCommandError]:
+            e = cls("msg", "p")
+            assert isinstance(e, PluginError)
+
+class TestPluginBridgeEnhancements:
+    def test_subscribe_plugin_registers_handler(self):
+        from bridges.plugin_bridge import PluginBridge
+        from bridges.core_bridge import CoreBridge
+        cb = CoreBridge()
+        pb = PluginBridge(cb)
+        pb.subscribe_plugin("test.topic", lambda msg: None, "test_plugin")
+        assert "test.topic" in pb.plugin_subscribers