`ssr.noExternal` and `environments.ssr.resolve.noExternal` are not merged correctly

[jamesopstad]

Describe the bug

If either ssr.noExternal or environments.ssr.resolve.noExternal are set to true then other values should be ignored and both should be resolved to true in the config. They are currently incorrectly merged to an array e.g. ['some-dep', true].

Reproduction

https://github.com/jamesopstad/vite-ssr-no-external-repro

Steps to reproduce

Run pnpm i followed by pnpm dev. Inspect vite.config.ts and logged output.

System Info

System:
    OS: macOS 15.5
    CPU: (12) arm64 Apple M3 Pro
    Memory: 258.20 MB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.2.0 - ~/.nvm/versions/node/v24.2.0/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v24.2.0/bin/yarn
    npm: 11.3.0 - ~/.nvm/versions/node/v24.2.0/bin/npm
    pnpm: 10.12.4 - ~/.nvm/versions/node/v24.2.0/bin/pnpm
  Browsers:
    Chrome: 138.0.7204.169
    Safari: 18.5

Used Package Manager

pnpm

Logs

No response

Validations

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• The provided reproduction is a minimal reproducible example of the bug.