`vp migrate` skips `tsdown` pack-config integration when Vite+ is already installed

[TheAlexLichter]

Describe the bug

Vite+ 0.3.1 migrates the import in a TypeScript tsdown config from tsdown to vite-plus/pack, but when vite-plus is already present in devDependencies, it does not create a vite.config.ts that exposes the config through pack. A subsequent vp pack silently ignores the configured entries and builds only the default src/index.ts entry.

The same fixture works when Vite+ is not preinstalled:migration creates vite.config.ts, imports the tsdown config, and vp pack builds both entries. This appears to be an uncovered partial-install migration case related to #1821 rather than a general tsdown migration failure.

Reproduction

https://github.com/TheAlexLichter/vite-plus-reproduction/tree/migrate-tsdown-config-ignored

Steps to reproduce

git clone --single-branch --branch migrate-tsdown-config-ignored https://github.com/TheAlexLichter/vite-plus-reproduction.git
cd vite-plus-reproduction
pnpm install --frozen-lockfile
pnpm test

System Info

- `vite-plus`: 0.3.1
- `tsdown`: 0.12.9
- `typescript`: 5.9.2
- `pnpm`: 10.28.2
- Node.js: 24.21.0
- OS: macOS arm64

Used Package Manager

pnpm

Logs

Validations

• Read the Contributing Guidelines.
• Check that there isn't already an issue for the same bug.
• Confirm this is a Vite+ issue and not an upstream issue (Vite, Vitest, tsdown, Rolldown, or Oxc).
• The provided reproduction is a minimal reproducible example.