Cookie header encoding

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/interesting-babbage-2lwwy5

To Reproduce

open preview in new window
refresh page to ensure cookie is set
see log in terminal

Current vs. Expected behavior

Current - user_name=John%2FDoe
Expected - user_name=John/Doe

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4242
  Available CPU cores: 2
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 15.2.2-canary.0 // Latest available version is detected (15.2.2-canary.0).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.3.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Not sure

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

Now i need to do:

const requestHeaders = Object.fromEntries(headersList.entries());
if (requestHeaders.cookie) {
    requestHeaders.cookie = decodeURIComponent(requestHeaders.cookie);
}

Start with the linked CodeSandbox reproduction, open the preview in a new window, refresh to set the cookie, and inspect the terminal log. Trace how the next dev server reads and logs the cookie header; done means the slash remains unencoded without requiring decodeURIComponent in the user's request-header handling.