fix(@angular/build): correct Vitest builder watch mode execution

The experimental Vitest-based unit test builder did not always correctly re-run tests in watch mode.

This commit refactors the Vitest executor to provide more accurate watch mode behavior by:
- Using `createVitest` to create a persistent instance that can be programmatically controlled.
- Disabling Vitest's internal file watcher (`watch: null`) to make the Angular builder's watcher the single source of truth.
- Triggering a test run via `vitest.rerunTestSpecifications()` within the `execute` method after each incremental build is complete.

Author: clydin

packages/angular/build/src/builders/unit-test/runners/vitest/executor.ts

@@ -15,7 +15,7 @@ import type { InlineConfig, Vitest } from 'vitest/node';
 import { assertIsError } from '../../../../utils/error';
 import { loadEsmModule } from '../../../../utils/load-esm';
 import { toPosixPath } from '../../../../utils/path';
-import type { FullResult, IncrementalResult } from '../../../application/results';
+import { type FullResult, type IncrementalResult, ResultKind } from '../../../application/results';
 import { writeTestFiles } from '../../../karma/application_builder';
 import { NormalizedUnitTestBuilderOptions } from '../../options';
 import type { TestExecutor } from '../api';
@@ -40,10 +40,41 @@ export class VitestExecutor implements TestExecutor {
     await writeTestFiles(buildResult.files, this.outputPath);
 
     this.latestBuildResult = buildResult;
+
+    // Initialize Vitest if not already present.
     this.vitest ??= await this.initializeVitest();
+    const vitest = this.vitest;
+
+    if (buildResult.kind === ResultKind.Incremental) {
+      const addedFiles = buildResult.added.map((file) => path.join(this.outputPath, file));
+      const modifiedFiles = buildResult.modified.map((file) => path.join(this.outputPath, file));
+
+      if (addedFiles.length === 0 && modifiedFiles.length === 0) {
+        yield { success: true };
+
+        return;
+      }
+
+      // If new files are added, use `start` to trigger test discovery.
+      // Also pass modified files to `start` to ensure they are re-run.
+      if (addedFiles.length > 0) {
+        await vitest.start([...addedFiles, ...modifiedFiles]);
+      } else {
+        // For modified files only, use the more efficient `rerunTestSpecifications`
+        const specsToRerun = modifiedFiles.flatMap((file) => vitest.getModuleSpecifications(file));
+
+        if (specsToRerun.length > 0) {
+          modifiedFiles.forEach((file) => vitest.invalidateFile(file));
+          await vitest.rerunTestSpecifications(specsToRerun);
+        }
+      }
+    } else {
+      // For the first build, perform a full run to discover and cache all specs
+      await vitest.start();
+    }
 
     // Check if all the tests pass to calculate the result
-    const testModules = this.vitest.state.getTestModules();
+    const testModules = vitest.state.getTestModules();
 
     yield { success: testModules.every((testModule) => testModule.ok()) };
   }
@@ -53,7 +84,7 @@ export class VitestExecutor implements TestExecutor {
   }
 
   private async initializeVitest(): Promise<Vitest> {
-    const { codeCoverage, reporters, watch, workspaceRoot, setupFiles, browsers, debug } =
+    const { codeCoverage, reporters, workspaceRoot, setupFiles, browsers, debug, watch } =
       this.options;
     const { outputPath, projectName, latestBuildResult } = this;
 
@@ -69,7 +100,7 @@ export class VitestExecutor implements TestExecutor {
         'The `vitest` package was not found. Please install the package and rerun the test command.',
       );
     }
-    const { startVitest } = vitestNodeModule;
+    const { createVitest } = vitestNodeModule;
 
     // Setup vitest browser options if configured
     const browserOptions = setupBrowserConfiguration(
@@ -99,9 +130,8 @@ export class VitestExecutor implements TestExecutor {
         }
       : {};
 
-    return startVitest(
+    return createVitest(
       'test',
-      undefined /* cliFilters */,
       {
         // Disable configuration file resolution/loading
         config: false,
@@ -115,6 +145,11 @@ export class VitestExecutor implements TestExecutor {
         ...debugOptions,
       },
       {
+        server: {
+          // Disable the actual file watcher. The boolean watch option above should still
+          // be enabled as it controls other internal behavior related to rerunning tests.
+          watch: null,
+        },
         plugins: [
           {
             name: 'angular:project-init',