refactor(@angular/build): optimize Vitest plugin file loading and improve safety

This commit improves the performance of the Vitest plugin by reusing a single
`TextDecoder` instance for decoding memory files, avoiding repeated instantiation.

It also adds safety checks when accessing the resolved Vitest configuration to
prevent potential errors if the configuration is not yet fully initialized.

Author: clydin

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

@@ -179,9 +179,10 @@ export async function createVitestConfigPlugin(
   };
 }
 
+const textDecoder = new TextDecoder('utf-8');
 async function loadResultFile(file: ResultFile): Promise<string> {
   if (file.origin === 'memory') {
-    return new TextDecoder('utf-8').decode(file.contents);
+    return textDecoder.decode(file.contents);
   }
 
   return readFile(file.inputPath, 'utf-8');
@@ -266,7 +267,7 @@ export function createVitestPlugins(pluginOptions: PluginOptions): VitestPlugins
         if (entryPoint) {
           outputPath = entryPoint + '.js';
 
-          if (vitestConfig.coverage.enabled) {
+          if (vitestConfig?.coverage?.enabled) {
             // To support coverage exclusion of the actual test file, the virtual
             // test entry point only references the built and bundled intermediate file.
             // If vitest supported an "excludeOnlyAfterRemap" option, this could be removed completely.
@@ -289,7 +290,7 @@ export function createVitestPlugins(pluginOptions: PluginOptions): VitestPlugins
 
           const map = sourceMapText ? JSON.parse(sourceMapText) : undefined;
           if (map) {
-            adjustSourcemapSources(map, !vitestConfig.coverage.enabled, workspaceRoot, id);
+            adjustSourcemapSources(map, !vitestConfig?.coverage?.enabled, workspaceRoot, id);
           }
 
           return {