fix: workaround noExternal merging bug on Vite 6

[hi-ogawa]

Description

Resolves #8899

I think what's happening is that config.ssr.noExternal: ["something"] and config.environments.ssr.resolve.noExternal: true are getting merged to noExternal: ["something", true] right after Vitest's config hook. This PR workarounds it by clearing it out so they become noExternal: true.

Either we workaround on Vitest or backport vitejs/vite#20502 to Vite 6, which might be equally simple though.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.
• Please check Allow edits by maintainers to make review process faster. Note that this option is not available for repositories that are owned by Github organizations.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

[netlify]

✅ Deploy Preview for vitest-dev ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	342b3ee
🔍 Latest deploy log	https://app.netlify.com/projects/vitest-dev/deploys/690c5f52ee9e1700086a4521
😎 Deploy Preview	https://deploy-preview-8950--vitest-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[pkg-pr-new]

@vitest/browser

npm i https://pkg.pr.new/@vitest/browser@8950

@vitest/browser-playwright

npm i https://pkg.pr.new/@vitest/browser-playwright@8950

@vitest/browser-preview

npm i https://pkg.pr.new/@vitest/browser-preview@8950

@vitest/browser-webdriverio

npm i https://pkg.pr.new/@vitest/browser-webdriverio@8950

@vitest/coverage-istanbul

npm i https://pkg.pr.new/@vitest/coverage-istanbul@8950

@vitest/coverage-v8

npm i https://pkg.pr.new/@vitest/coverage-v8@8950

@vitest/expect

npm i https://pkg.pr.new/@vitest/expect@8950

@vitest/mocker

npm i https://pkg.pr.new/@vitest/mocker@8950

@vitest/pretty-format

npm i https://pkg.pr.new/@vitest/pretty-format@8950

@vitest/runner

npm i https://pkg.pr.new/@vitest/runner@8950

@vitest/snapshot

npm i https://pkg.pr.new/@vitest/snapshot@8950

@vitest/spy

npm i https://pkg.pr.new/@vitest/spy@8950

@vitest/ui

npm i https://pkg.pr.new/@vitest/ui@8950

@vitest/utils

npm i https://pkg.pr.new/@vitest/utils@8950

vitest

npm i https://pkg.pr.new/vitest@8950

@vitest/web-worker

npm i https://pkg.pr.new/@vitest/web-worker@8950

@vitest/ws-client

npm i https://pkg.pr.new/@vitest/ws-client@8950

commit: 0af4a14

[sheremet-va]

LGTM, I think the fix for Vite 6 can also be bakported

[ascorbic]

Thanks @hi-ogawa