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: false`) to make the Angular builder's watcher the single source of truth.
- Triggering a test run via `vitest.start()` within the `execute` method after each incremental build is complete.

Author: clydin

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

@@ -40,8 +40,13 @@ 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();
 
+    // Trigger a test run
+    await this.vitest.start();
+
     // Check if all the tests pass to calculate the result
     const testModules = this.vitest.state.getTestModules();
 
@@ -69,7 +74,7 @@ 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 +104,8 @@ export class VitestExecutor implements TestExecutor {
         }
       : {};
 
-    return startVitest(
+    return createVitest(
       'test',
-      undefined /* cliFilters */,
       {
         // Disable configuration file resolution/loading
         config: false,
@@ -110,7 +114,7 @@ export class VitestExecutor implements TestExecutor {
         name: 'base',
         include: [],
         reporters: reporters ?? ['default'],
-        watch,
+        watch: false,
         coverage: generateCoverageOption(codeCoverage, workspaceRoot, this.outputPath),
         ...debugOptions,
       },