SSR transform drops `map.sources` when multiple `sources`

[AriPerkkio]

Describe the bug

When Vite SSR transforms file that has source map with multiple sources, only a single entry is returned in the final source map's sources. This causes issues in Vitest: vitest-dev/vitest#6120

With source map like

{
  "version": 3,
  "file": "bundle.js",
  "sources": [
    "./first.ts",
    "./second.ts"
  ],
  "sourcesContent": [
    "...",
    "..."
  ],
  "names": [
    "covered",
    "uncovered"
  ],
  "mappings": "..."
}

... Vite returns map with sources: ["./first.ts"]. The "./second.ts" is dropped during SSR transform.

Reproduction

https://stackblitz.com/~/edit/vitejs-vite-fj3cj8

Steps to reproduce

pnpm install
pnpm run build # Create a bundle with Rollup
pnpm run start # Calls "node ./server.mjs" which runs Vite via Node API

const ssr = await server.transformRequest("./dist/bundle.js", { ssr: true });
const web = await server.transformRequest("./dist/bundle.js", { ssr: false });

console.log({ ssr: ssr.map.sources, web: web.map.sources });

{
  ssr: [ '../src/first.ts' ], // <-- Missing '../src/second.ts'
  web: [ '../src/first.ts', '../src/second.ts' ]
}

System Info

System:
    OS: macOS 14.5
    CPU: (8) arm64 Apple M2
    Memory: 74.52 MB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 20.15.0 - ~/.nvm/versions/node/v20.15.0/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v20.15.0/bin/yarn
    npm: 10.7.0 - ~/.nvm/versions/node/v20.15.0/bin/npm
    pnpm: 9.4.0 - ~/.nvm/versions/node/v20.15.0/bin/pnpm
  Browsers:
    Chrome: 126.0.6478.127
    Safari: 17.5
  npmPackages:
    @vitejs/release-scripts: ^1.3.1 => 1.3.1 
    rollup: ^4.13.0 => 4.13.0 
    vite: workspace:* => 5.3.3

Used Package Manager

pnpm

Logs

No response

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