Some fetch operations fail with testProxy enabled

### Link to the code that reproduces this issue https://github.com/dustinsoftware/next-testmode-issue-repro ### To Reproduce USE_TEST_PROXY=1 BUCKET_NAME=bucket-name npm run dev ### Current vs. Expected behavior Having the testProxy mode enabled in next config results in some fetch operations failing when the app is run in dev mode and in standalone mode. This is similar to https://github.com/vercel/next.js/issues/66238 Impacted fetch calls fail with the following stack: ``` {"level":50,"time":1755810957931,"pid":10975,"hostname":"xxx","name":"console","err":{"type":"TypeError","message":"fetch failed: expect header not supported","stack":"TypeError: fetch failed\n at node:internal/deps/undici/undici:13510:13\n at process.processTicksAndRejections (node:internal/process/task_queues:105:5)\n at async /Users/dustin.masters/Code/next-aws-sdk-minimal-repro/.next/standalone/node_modules/next/dist/experimental/testmode/httpget.js:16:26\n at async s.push.done (/Users/dustin.masters/Code/next-aws-sdk-minimal-repro/.next/standalone/node_modules/next/dist/compiled/@mswjs/interceptors/ClientRequest/index.js:1:15030)\ncaused by: NotSupportedError: expect header not supported\n at processHeader (node:internal/deps/undici/undici:2174:15)\n at new Request (node:internal/deps/undici/undici:1998:15)\n at [dispatch] (node:internal/deps/undici/undici:7802:25)\n at Intercept (node:internal/deps/undici/undici:7535:20)\n at [Intercepted Dispatch] (node:internal/deps/undici/undici:572:16)\n at Client.dispatch (node:internal/deps/undici/undici:588:44)\n at [dispatch] (node:internal/deps/undici/undici:819:32)\n at [Intercepted Dispatch] (node:internal/deps/undici/undici:565:33)\n at Pool.dispatch (node:internal/deps/undici/undici:588:44)\n at [dispatch] (node:internal/deps/undici/undici:8230:27)","$metadata":{"attempts":1,"totalRetryDelay":0}},"msg":"fetch failed"} ``` ### Provide environment information ```bash Operating System: Platform: darwi…