feat: 插件自动挂载测试机制 — capabilities + self_test + 3阶段启动

- Plugin trait 增加 capabilities() 和 self_test() 方法
- PluginVTable 增加 get_capabilities 和 self_test FFI
- ServiceManager 三阶段启动: init → self_test → start
- SendCallback 改为 ctx 参数传递，消除 thread_local
- export_plugin! 宏所有 FFI 函数包裹 catch_unwind
- PluginManifest 增加 capabilities/required_capabilities/auto_test
- 新增 3 个自测相关测试用例 (共 59 测试)

plugins/example-plugin/src/lib.rs

@@ -1,18 +1,23 @@
 //! 示例动态插件 — 展示如何使用 showen-plugin-sdk 编写插件
 //!
-//! 此插件仅打印日志，用于验证动态加载流程。
+//! 此插件演示动态加载流程及自测机制。
 
 use showen_plugin_sdk::{
-    export_plugin, Message, MessageSender, PluginInfo, ShowenPlugin,
+    export_plugin, CapabilityTestResult, Message, MessageSender, PluginInfo, ShowenPlugin,
 };
 
 pub struct ExamplePlugin {
     sender: Option<MessageSender>,
+    /// 用于演示可配置的自测失败
+    fail_optional_test: bool,
 }
 
 impl ExamplePlugin {
     pub fn new() -> Self {
-        Self { sender: None }
+        Self {
+            sender: None,
+            fail_optional_test: false,
+        }
     }
 }
 
@@ -26,6 +31,29 @@ impl ShowenPlugin for ExamplePlugin {
         }
     }
 
+    fn capabilities(&self) -> Vec<String> {
+        vec!["logging".to_string(), "metrics".to_string()]
+    }
+
+    fn self_test(&mut self) -> Vec<CapabilityTestResult> {
+        vec![
+            CapabilityTestResult {
+                capability: "logging".to_string(),
+                passed: true,
+                message: "log output verified".to_string(),
+            },
+            CapabilityTestResult {
+                capability: "metrics".to_string(),
+                passed: !self.fail_optional_test,
+                message: if self.fail_optional_test {
+                    "metrics backend unreachable".to_string()
+                } else {
+                    "metrics endpoint ok".to_string()
+                },
+            },
+        ]
+    }
+
     fn init(&mut self, config_json: &str, sender: MessageSender) -> Result<(), String> {
         eprintln!("[ExamplePlugin] init called, config length: {}", config_json.len());
         self.sender = Some(sender);