fix(mcp): resolve compilation errors and add backward compatibility

## Compilation Fixes
- Add deprecated compatibility methods to McpService:
  * get_servers() - filters servers by app
  * set_enabled() - delegates to toggle_app()
  * sync_enabled() - syncs enabled servers for specific app
  * import_from_claude/codex/gemini() - wraps mcp:: functions
- Fix toml_edit type conversion in sync_single_server_to_codex():
  * Add json_server_to_toml_table() helper function
  * Manually construct toml_edit::Table instead of invalid serde conversion
- Fix get_codex_config_path() calls (returns PathBuf, not Result)
- Update upsert_mcp_server_in_config() to work with unified structure:
  * Converts old per-app API to unified McpServer structure
  * Preserves existing server data when updating
  * Supports sync_other_side parameter for multi-app enable
- Update delete_mcp_server_in_config() to ignore app parameter

## Backward Compatibility
- All old v3.6.x commands continue to work with deprecation warnings
- Frontend migration can be done incrementally
- Old commands transparently use new unified backend

## Status
✅ Backend compiles successfully (cargo check passes)
⚠️ 16 warnings (8 deprecation + 8 unused functions - expected)

src-tauri/src/services/mcp.rs

@@ -186,4 +186,82 @@ impl McpService {
 
         Ok(())
     }
+
+    // ========================================================================
+    // 兼容层：支持旧的 v3.6.x 命令（已废弃，将在 v4.0 移除）
+    // ========================================================================
+
+    /// [已废弃] 获取指定应用的 MCP 服务器（兼容旧 API）
+    #[deprecated(since = "3.7.0", note = "Use get_all_servers instead")]
+    pub fn get_servers(
+        state: &AppState,
+        app: AppType,
+    ) -> Result<HashMap<String, serde_json::Value>, AppError> {
+        let all_servers = Self::get_all_servers(state)?;
+        let mut result = HashMap::new();
+
+        for (id, server) in all_servers {
+            if server.apps.is_enabled_for(&app) {
+                result.insert(id, server.server);
+            }
+        }
+
+        Ok(result)
+    }
+
+    /// [已废弃] 设置 MCP 服务器在指定应用的启用状态（兼容旧 API）
+    #[deprecated(since = "3.7.0", note = "Use toggle_app instead")]
+    pub fn set_enabled(
+        state: &AppState,
+        app: AppType,
+        id: &str,
+        enabled: bool,
+    ) -> Result<bool, AppError> {
+        Self::toggle_app(state, id, app, enabled)?;
+        Ok(true)
+    }
+
+    /// [已废弃] 同步启用的 MCP 到指定应用（兼容旧 API）
+    #[deprecated(since = "3.7.0", note = "Use sync_all_enabled instead")]
+    pub fn sync_enabled(state: &AppState, app: AppType) -> Result<(), AppError> {
+        let servers = Self::get_all_servers(state)?;
+
+        for server in servers.values() {
+            if server.apps.is_enabled_for(&app) {
+                Self::sync_server_to_app(state, server, &app)?;
+            }
+        }
+
+        Ok(())
+    }
+
+    /// [已废弃] 从 Claude 导入 MCP（兼容旧 API）
+    #[deprecated(since = "3.7.0", note = "Import will be handled differently in unified structure")]
+    pub fn import_from_claude(state: &AppState) -> Result<usize, AppError> {
+        let mut cfg = state.config.write()?;
+        let count = mcp::import_from_claude(&mut cfg)?;
+        drop(cfg);
+        state.save()?;
+        Ok(count)
+    }
+
+    /// [已废弃] 从 Codex 导入 MCP（兼容旧 API）
+    #[deprecated(since = "3.7.0", note = "Import will be handled differently in unified structure")]
+    pub fn import_from_codex(state: &AppState) -> Result<usize, AppError> {
+        let mut cfg = state.config.write()?;
+        let count = mcp::import_from_codex(&mut cfg)?;
+        drop(cfg);
+        state.save()?;
+        Ok(count)
+    }
+
+    /// [已废弃] 从 Gemini 导入 MCP（兼容旧 API）
+    #[deprecated(since = "3.7.0", note = "Import will be handled differently in unified structure")]
+    pub fn import_from_gemini(state: &AppState) -> Result<usize, AppError> {
+        let mut cfg = state.config.write()?;
+        let count = mcp::import_from_gemini(&mut cfg)?;
+        drop(cfg);
+        state.save()?;
+        Ok(count)
+    }
 }