test: verify snapshot updates in the Vitest snapshot e2e test

tests/e2e/tests/vitest/snapshot.ts

@@ -1,4 +1,4 @@
-import { ng } from '../../utils/process';
+import { execWithEnv, ng } from '../../utils/process';
 import { replaceInFile, readFile, writeFile } from '../../utils/fs';
 import { applyVitestBuilder } from '../../utils/vitest';
 import assert from 'node:assert/strict';
@@ -76,4 +76,50 @@ describe('App', () => {
     },
     'Snapshot mismatch did not cause the test to fail.',
   );
+
+  // Fourth run: update the snapshots to match the new output.
+  // Vitest reads the `UPDATE_SNAPSHOT` environment variable directly. The builder does not
+  // currently forward Vitest's `--update` flag, so the variable is the way to request an update
+  // without a Vitest configuration file.
+  const { stdout: updateRunStdout } = await execWithEnv('ng', ['test'], {
+    ...process.env,
+    UPDATE_SNAPSHOT: 'all',
+  });
+  assert.match(
+    stripVTControlCharacters(updateRunStdout),
+    /Snapshots\s+2 updated/,
+    'Snapshots were not updated when UPDATE_SNAPSHOT=all was set.',
+  );
+
+  const updatedSpecContent = await readFile('src/app/app.spec.ts');
+  assert.match(
+    updatedSpecContent,
+    /toMatchInlineSnapshot\(`"Snapshot is broken!"`\)/,
+    'Inline snapshot was not updated in the spec file.',
+  );
+  assert.doesNotMatch(
+    updatedSpecContent,
+    /toMatchInlineSnapshot\(`"test-project"`\)/,
+    'Stale inline snapshot is still present in the spec file.',
+  );
+
+  const updatedSnapshotContent = await readFile('src/app/__snapshots__/app.spec.ts.snap');
+  assert.match(
+    updatedSnapshotContent,
+    /exports\[`App > should match file snapshot 1`\] = `"Snapshot is broken!"`;/,
+    'File snapshot was not updated on disk.',
+  );
+  assert.doesNotMatch(
+    updatedSnapshotContent,
+    /"test-project"/,
+    'Stale file snapshot is still present on disk.',
+  );
+
+  // Fifth run: tests should pass with the updated snapshots and no snapshot should be rewritten
+  const { stdout: finalRunStdout } = await ng('test');
+  assert.doesNotMatch(
+    stripVTControlCharacters(finalRunStdout),
+    /Snapshots\s+\d+ (written|updated|failed|obsolete)/,
+    'Snapshots were unexpectedly modified after being updated.',
+  );
 }