fix(@angular/build): suggest the istanbul provider in the unit-test V8 browser coverage error

When code coverage uses the V8 provider with a non-Chromium browser, the unit-test builder told
users to disable coverage or remove the browser. The error predates Istanbul coverage support in
the Vitest runner, so it never mentioned the option that keeps both. Suggest the `istanbul` provider
first. The browser coverage e2e test now asserts the new hint.

Author: MeAkib

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

@@ -529,7 +529,8 @@ function validateBrowserCoverage(
       `Code coverage is enabled, but the following configured browsers do not support the V8 coverage provider: ` +
         `${unsupportedBrowsers.join(', ')}. ` +
         `V8 coverage is only supported on Chromium-based browsers (e.g., Chrome, Chromium, Edge). ` +
-        `Please disable coverage or remove the unsupported browsers.`,
+        `Please use the "istanbul" coverage provider (requires "@vitest/coverage-istanbul"), ` +
+        `remove the unsupported browsers, or disable coverage.`,
     );
   }
 }

tests/e2e/tests/vitest/browser-coverage-validation.ts

@@ -67,6 +67,11 @@ export default async function (): Promise<void> {
       /Code coverage is enabled, but the following configured browsers do not support the V8 coverage provider: firefox/,
       'Expected validation error for unsupported browser with coverage (config name).',
     );
+    assert.match(
+      output2,
+      /Please use the "istanbul" coverage provider/,
+      'Expected the error to suggest the istanbul coverage provider.',
+    );
 
     // === Case 3: Browser configured via vitest.config.mts (instances) ===
     await writeFile(