fix(desktop): Tauri 端找碴验证 7 项修复 — 消息泄漏/UUID暴露/错误友好化

B15/B11: streamStore onAgentStream 添加 activeRunId 过滤，移除降级匹配，
hand/workflow 消息追加前验证 runId 归属；chatStore 切换/新建对话时
先 cancelStream 终止旧流；ChatArea hand-execution-complete 事件
添加 isStreaming 守卫

B4/B5: ChatArea 模型列表过滤 embedding 模型，provider 设为 undefined
隐藏 UUID

B2/B3: streamStore onError 添加 formatUserError 函数，将原始 JSON
错误转换为中文友好提示

B1: SuggestionChips onSelect 延迟调用 handleSend 自动发送建议

fix(runtime): test_util.rs with_error 添加 mut self，with_stream_chunks
移除多余 mut

fix(saas): lib.rs 添加 Result/SaasError re-export

desktop/src/store/chatStore.ts

@@ -155,12 +155,18 @@ export const useChatStore = create<ChatState>()(
   },
 
   newConversation: () => {
+    if (get().isStreaming) {
+      useStreamStore.getState().cancelStream();
+    }
     const messages = get().messages;
     useConversationStore.getState().newConversation(messages);
     set({ messages: [], isStreaming: false });
   },
 
   switchConversation: (id: string) => {
+    if (get().isStreaming) {
+      useStreamStore.getState().cancelStream();
+    }
     const messages = get().messages;
     const result = useConversationStore.getState().switchConversation(id, messages);
     if (result) {