fix(cli): add helpful error message for missing jest dependency

Since jest is now a peer dependencies, users must add it as a
devDependency themselves. This commit adds early detection and
a helpful error message when jest is missing, directing users
to the migration guide.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

packages/cli/src/modules/test/commands/package/test.ts

@@ -83,5 +83,23 @@ export default async (_opts: OptionValues, cmd: Command) => {
     (process.stdout as any)._handle.setBlocking(true);
   }
 
+  // Because of the ongoing migration to v30 of jest, jest is no longer hard-depended to allow
+  // opt-in migration. Users instead need to add jest as a devDependency themselves and specify
+  // the version they want. This prints a helpful error message if jest is not found, i.e. they
+  // forgot/didn't know they had to add jest themselves.
+  try {
+    require.resolve('jest');
+  } catch {
+    console.error(
+      [
+        'No Jest installation found in this project.',
+        '',
+        'To support opt-in migration to Jest v30, the Backstage CLI now expects Jest to be installed as a devDependency.',
+        'See the migration guide in the changelog for version options and their consequences.',
+      ].join('\n'),
+    );
+    process.exit(1);
+  }
+
   await require('jest').run(args);
 };