fix: don't call next middleware if user sent response in proxy.bypass (#19318)

Author: sapphi-red

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

@@ -225,6 +225,9 @@ export function proxyMiddleware(
             if (typeof bypassResult === 'string') {
               debug?.(`bypass: ${req.url} -> ${bypassResult}`)
               req.url = bypassResult
+              if (res.writableEnded) {
+                return
+              }
               return next()
             }
             if (bypassResult === false) {