Angular build fails to resolve CSS when package is externalized

### Which @angular/* package(s) are the source of the bug?

compiler

### Is this a regression?

No

### Description

Hi there 👋

In a micro-frontend architecture, our goal is to compile CSS utilities while relying on importmap to load all JavaScript. I'm not sure if I’ve missed a configuration step or if this is expected behavior.

However, in the Vite ecosystem, when using the option to externalize dependencies, all the CSS is correctly resolved, but the JavaScript is excluded from the import.

```js
export default defineConfig((env) => {
return {
build: {
rollupOptions: {
external: ["bootstrap"],
},
},
});
```

However, in the Angular environment, this approach causes the build to fail because it can't locate the CSS.

```json
"projects": {
"test-angular": {
"architect": {
"build": {
"builder": "@angular-devkit/build-angular:application",
"options": {
"externalDependencies": ["bootstrap"],
```

To isolate the issue, I created a minimal reproduction using Bootstrap.

Thank you very much for the awesome work 🚀

### Please provide a link to a minimal reproduction of the bug

https://github.com/Tragio/angular-externalize-repro

### Please provide the exception or error you saw

```true
> ng build

Application bundle generation failed. [1.968 seconds]

✘ [ERROR] Can't find stylesheet to import.
╷
2 │ @use "bootstrap/scss/bootstrap";
│ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
╵
src/styles.scss 2:1 root stylesheet [plugin angular-sass]

angular:styles/global:styles:1:8:
1 │ @import 'src/styles.scss';
╵ ~~~~~~~~~~~~~~~~~
```

### Please provide the environment you discovered this bug in (run `ng version`)

```true
Angular CLI: 19.2.14
Node: 22.14.0
Package Manager: npm 10.9.2
OS: darwin arm64

Angular: 19.2.14
... cli, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router

Package Version
---------------------------------------------------------
@angular-devkit/architect 0.1902.14
@angular-devkit/build-angular 19.2.14
@angular-devkit/core 19.2.14
@angular-devkit/schematics 19.2.14
@schematics/angular 19.2.14
rxjs 7.8.2
typescript 5.7.3
zone.js 0.15.1
```

### Anything else?

_No response_

Bắt đầu với bản tái hiện tối thiểu được liên kết, đặc biệt là angular.json và src/styles.scss, rồi chạy ng build với bootstrap được liệt kê trong externalDependencies. Theo dõi cách các dependency được externalize được xử lý trong quá trình build Sass/global-styles của Angular; hoàn tất khi JavaScript vẫn ở bên ngoài trong khi bootstrap/scss/bootstrap được resolve thành công.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.