fix(@angular/build): serve emitted HTML assets in dev-server

Previously, `assets-middleware` excluded all `.html` files from being served from virtual output files, causing servable HTML files (such as those emitted via `file` loader when imported from TypeScript) to fall through to `html-fallback-middleware` and redirect to `/index.html`.

This commit updates the check in `assets-middleware` so that servable HTML assets (excluding `/index.html` and `/index.csr.html`) are served directly.

Fixes angular#34166

Author: alan-agius4

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

@@ -235,5 +235,30 @@ describeServeBuilder(executeDevServer, DEV_SERVER_BUILDER_INFO, (harness, setupT
       expect(result?.success).toBeTrue();
       expect(response?.status).toBe(404);
     });
+
+    it('serves an HTML asset emitted via file loader', async () => {
+      const htmlFileContent = '<h1>Custom HTML Asset</h1>';
+      await harness.writeFiles({
+        'src/test.html': htmlFileContent,
+        'src/types.d.ts': 'declare module "*.html" { const url: string; export default url; }',
+        'src/main.ts': 'import testHtml from "./test.html";\n console.log(testHtml);',
+      });
+
+      setupTarget(harness, {
+        loader: {
+          '.html': 'file',
+        },
+      });
+
+      harness.useTarget('serve', {
+        ...BASE_OPTIONS,
+      });
+
+      const { result, response } = await executeOnceAndFetch(harness, 'media/test.html');
+      expect(result?.success).toBeTrue();
+      expect(response?.status).toBe(200);
+      expect(response?.headers.get('content-type')).toContain('text/html');
+      expect(await response?.text()).toBe(htmlFileContent);
+    });
   });
 });

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

@@ -98,7 +98,7 @@ export function createAngularAssetsMiddleware(
     // Resource files are handled directly.
     // Global stylesheets (CSS files) are currently considered resources to workaround
     // dev server sourcemap issues with stylesheets.
-    if (extension !== '.js' && extension !== '.html') {
+    if (extension !== '.js' && pathname !== '/index.html' && pathname !== '/index.csr.html') {
       const outputFile = outputFiles.get(pathname);
       if (outputFile?.servable) {
         let data: Uint8Array | string = outputFile.contents;