#23506 mergeConfig crashes with TypeError when server.ws is false and server.hmr is merged

Describe the bug

mergeConfig throws an uncaught TypeError when merging a server.hmr object while server.ws is set to false:

TypeError: Property description must be an object: undefined

This also happens during normal config resolution whenever a plugin's config/configResolved hook (or a second loadConfigFromFile result) returns an hmr object on top of a base config that has server.ws: false.

Root cause: in mergeConfigRecursively, when the recursion reaches server, setupHmrWsOptionCompat is called. When serverConfig.ws === false, that helper returns early without defining the wsOptionKeys accessors on the hmr object. Later, when recursion reaches server.hmr (base and override both have an hmr object), mergeConfigRecursively unconditionally does:

Object.defineProperty(
  merged,
  key,
  Object.getOwnPropertyDescriptor(defaults, key)!,
)

Object.getOwnPropertyDescriptor(defaults, key) returns undefined because the accessors were never installed, and the non-null assertion turns into Object.defineProperty(merged, key, undefined) → TypeError.

I have a fix ready and will open a PR.

Reproduction

https://gist.github.com/scs0209/67329ba3aa7e6ff9e12a359e696e0336

repro.mjs:

import { mergeConfig } from 'vite'

const base = { server: { ws: false, hmr: { host: 'localhost' } } }
const override = { server: { hmr: { port: 5173 } } }

// throws:
// TypeError: Property description must be an object: undefined
console.log(mergeConfig(base, override))

Steps to reproduce

npm install a project with vite (>= 6, reproduced on v8.3.0 and main)
Run the snippet above, or start a dev server where a plugin's config hook returns:
```
config() {
  return { server: { ws: false } }
}
```
while another plugin's config hook returns { server: { hmr: { host: '...' } } }
Observe the TypeError instead of a merged config

System Info

Vite: reproduced on v8.3.0 and current main
Node.js: v26.0.0
npm: 11.12.1
OS: macOS (arm64)