@import "tailwindcss" not processed in build output when using tsup with Tailwind CSS v4

Hi! I recently needed to use tsup to build a test application for a UI kit that uses Tailwind CSS v4. However, when I ran the build command with the configuration below, the `@import "tailwindcss"` statement in my index.css file was not processed. It remained as-is in the final build output, which caused none of my component styles to be applied.

Here’s the tsup configuration I’m using:

```ts
import { defineConfig } from 'tsup';

export default defineConfig({
entry: ['src/index.ts'],
format: ['cjs', 'esm'],
outDir: 'dist',
tsconfig: 'tsconfig.build.json',
minify: true,
sourcemap: false,
clean: true,
dts: true,
});
```

Project stack
- Build tool: tsup
- Framework: Vite + React
- Language: TypeScript
- CSS: Tailwind CSS v4

Let me know if you'd like me to provide a minimal reproduction repo. Thanks in advance!

Reproduce the build using the shown tsup configuration, src/index.ts, and the index.css file containing @import "tailwindcss". Check the final dist output first; done means the import is processed during the build and the component styles are applied rather than leaving the statement unchanged.

Written by the indexing model from the issue text.