fix(dev): return "502 Bad Gateway" on proxy failures instead of 500 (#21652)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: sapphi-red <49056869+sapphi-red@users.noreply.github.com>

Author: Copilot

packages/vite/src/node/server/middlewares/proxy.ts

@@ -106,7 +106,7 @@ export function proxyMiddleware(
         )
         if (!res.headersSent && !res.writableEnded) {
           res
-            .writeHead(500, {
+            .writeHead(502, {
               'Content-Type': 'text/plain',
             })
             .end()

playground/proxy-bypass/__tests__/proxy-bypass.spec.ts

@@ -1,5 +1,5 @@
 import { expect, test, vi } from 'vitest'
-import { browserLogs, page, serverLogs } from '~utils'
+import { browserLogs, isServe, page, serverLogs, viteTestUrl } from '~utils'
 
 test('proxy-bypass', async () => {
   await vi.waitFor(() => {
@@ -17,3 +17,8 @@ test('async-proxy-bypass-with-error', async () => {
     expect(serverLogs.join('\n')).toContain('bypass error')
   })
 })
+
+test.runIf(isServe)('proxy error returns 502', async () => {
+  const res = await fetch(viteTestUrl + '/proxyError')
+  expect(res.status).toBe(502)
+})

playground/proxy-bypass/vite.config.js

@@ -12,6 +12,9 @@ export default defineConfig({
           return false
         },
       },
+      '/proxyError': {
+        target: 'http://foo.invalid',
+      },
       '/asyncResponse': {
         bypass: async (_, res) => {
           await timeout(4)