fix(@angular/build): skip vite transformation of CSS-like assets

This change ensures that Vite's CSS transformation is only applied to actual stylesheets and not assets.

Closes #30792

Author: alan-agius4

packages/angular/build/src/builders/dev-server/tests/behavior/build-assets_spec.ts

@@ -53,6 +53,42 @@ describeServeBuilder(executeDevServer, DEV_SERVER_BUILDER_INFO, (harness, setupT
       expect(await response?.text()).toContain(javascriptFileContent);
     });
 
+    it('serves a project CSS asset unmodified', async () => {
+      const cssFileContent = 'p { color: blue };';
+      await harness.writeFile('src/extra.css', cssFileContent);
+
+      setupTarget(harness, {
+        assets: ['src/extra.css'],
+      });
+
+      harness.useTarget('serve', {
+        ...BASE_OPTIONS,
+      });
+
+      const { result, response } = await executeOnceAndFetch(harness, 'extra.css');
+
+      expect(result?.success).toBeTrue();
+      expect(await response?.text()).toBe(cssFileContent);
+    });
+
+    it('serves a project SCSS asset unmodified', async () => {
+      const cssFileContent = 'p { color: blue };';
+      await harness.writeFile('src/extra.scss', cssFileContent);
+
+      setupTarget(harness, {
+        assets: ['src/extra.scss'],
+      });
+
+      harness.useTarget('serve', {
+        ...BASE_OPTIONS,
+      });
+
+      const { result, response } = await executeOnceAndFetch(harness, 'extra.scss');
+
+      expect(result?.success).toBeTrue();
+      expect(await response?.text()).toBe(cssFileContent);
+    });
+
     it('should return 404 for non existing assets', async () => {
       setupTarget(harness, {
         assets: [],

packages/angular/build/src/tools/vite/middlewares/assets-middleware.ts

@@ -20,6 +20,7 @@ export interface ComponentStyleRecord {
   reload?: boolean;
 }
 
+const CSS_PREPROCESSOR_REGEXP = /\.(?:s[ac]ss|less|css)$/;
 const JS_TS_REGEXP = /\.[cm]?[tj]sx?$/;
 
 export function createAngularAssetsMiddleware(
@@ -43,8 +44,8 @@ export function createAngularAssetsMiddleware(
     // Rewrite all build assets to a vite raw fs URL
     const asset = assets.get(pathname);
     if (asset) {
-      // This is a workaround to serve JS and TS files without Vite transformations.
-      if (JS_TS_REGEXP.test(extension)) {
+      // This is a workaround to serve CSS, JS and TS files without Vite transformations.
+      if (JS_TS_REGEXP.test(extension) || CSS_PREPROCESSOR_REGEXP.test(extension)) {
         const contents = readFileSync(asset.source);
         const etag = `W/${createHash('sha256').update(contents).digest('hex')}`;
         if (checkAndHandleEtag(req, res, etag)) {