fix(pda): preserve packing plan retry keys and surface missing API

tests/return-packing-api.test.mjs

@@ -1,6 +1,6 @@
 import test from 'node:test'
 import assert from 'node:assert/strict'
-import { findPackingOrders, getReturnCartonLabel, mergeReturnCartons, normalizePackingPreview } from '../src/api/return-packing.js'
+import { findPackingOrders, getReturnCartonLabel, listReturnPackingPlans, mergeReturnCartons, normalizePackingPreview } from '../src/api/return-packing.js'
 import { session } from '../src/stores/session.js'
 
 test('packing preview keeps customer scope internal and exposes stable order references', () => {
@@ -40,3 +40,11 @@ test('return packing lookup still rejects real service errors', async () => {
 	globalThis.uni = { request(options) { options.success({ statusCode: 500, data: { code: 'B0500', message: '服务暂不可用' }, header: {} }) } }
 	await assert.rejects(() => findPackingOrders('RT-1'), (error) => error.code === 'B0500')
 })
+
+test('plan list distinguishes an empty success from a missing API and preserves diagnostics', async () => {
+	session.apiBase = '/api'; session.token = 'packing-token'
+	globalThis.uni = { request(options) { options.success({ statusCode: 200, data: { code: '200', data: [] }, header: {} }) } }
+	assert.deepEqual(await listReturnPackingPlans(), [])
+	globalThis.uni = { request(options) { options.success({ statusCode: 404, data: { code: 'A0404', message: '计划接口不存在', requestId: 'missing-plan-api' }, header: {} }) } }
+	await assert.rejects(() => listReturnPackingPlans(), error => error.code === 'A0404' && error.requestId === 'missing-plan-api')
+})