fix(@angular/build): add asset tracking to application builder watch files

This commit updates the application builder to include assets in the watch process, triggering file re-copying when changes are detected.

Closes #28415

Author: alan-agius4

packages/angular/build/src/builders/application/tests/behavior/rebuild-assets_spec.ts

@@ -0,0 +1,65 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { concatMap, count, take, timeout } from 'rxjs';
+import { buildApplication } from '../../index';
+import { APPLICATION_BUILDER_INFO, BASE_OPTIONS, describeBuilder } from '../setup';
+
+/**
+ * Maximum time in milliseconds for single build/rebuild
+ * This accounts for CI variability.
+ */
+const BUILD_TIMEOUT = 10_000;
+
+describeBuilder(buildApplication, APPLICATION_BUILDER_INFO, (harness) => {
+  describe('Behavior: "Rebuilds when input asset changes"', () => {
+    beforeEach(async () => {
+      // Application code is not needed for styles tests
+      await harness.writeFile('src/main.ts', 'console.log("TEST");');
+      await harness.writeFile('public/asset.txt', 'foo');
+    });
+
+    it('emits updated asset', async () => {
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        assets: [
+          {
+            glob: '**/*',
+            input: 'public',
+          },
+        ],
+        watch: true,
+      });
+
+      const buildCount = await harness
+        .execute({ outputLogsOnFailure: false })
+        .pipe(
+          timeout(BUILD_TIMEOUT),
+          concatMap(async ({ result }, index) => {
+            switch (index) {
+              case 0:
+                expect(result?.success).toBeTrue();
+                harness.expectFile('dist/browser/asset.txt').content.toContain('foo');
+
+                await harness.writeFile('public/asset.txt', 'bar');
+                break;
+              case 1:
+                expect(result?.success).toBeTrue();
+                harness.expectFile('dist/browser/asset.txt').content.toContain('bar');
+                break;
+            }
+          }),
+          take(2),
+          count(),
+        )
+        .toPromise();
+
+      expect(buildCount).toBe(2);
+    });
+  });
+});

packages/angular/build/src/tools/esbuild/bundler-execution-result.ts

@@ -70,6 +70,7 @@ export class ExecutionResult {
 
   addAssets(assets: BuildOutputAsset[]): void {
     this.assetFiles.push(...assets);
+    this.extraWatchFiles.push(...assets.map(({ source }) => source));
   }
 
   addLog(value: string): void {