修复: FR-S3 approve decision 非空校验 + FR-R1 switchConversation 切换 token

- FR-S3: approve_human_approval 加 decision 非空校验(防 "" 透传;HumanNode options 非空时还校验 ∈ options)
- FR-R1: switchConversation 加 _latestSwitchId token,过期响应丢弃(防快速连点 A→B 后返回的 A 覆盖 B messages)
来源 fullstack-review §2/§4(核实确认);cargo/vue-tsc 0 err

src-tauri/src/commands/workflow.rs

@@ -185,6 +185,10 @@ pub async fn approve_human_approval(
     decision: String,
     comment: Option<String>,
 ) -> Result<(), String> {
+    // decision 非空校验(FR-S3: 防 "" 透传;HumanNode options 非空时还校验 ∈ options)
+    if decision.trim().is_empty() {
+        return Err("审批决策不能为空".to_string());
+    }
     // 发送审批响应到事件总线
     let response = HumanApprovalResponse {
         execution_id: execution_id.clone(),