test(node-core): Fix flaky cron instrumentation test (#21092)

## Summary

`suites/cron/cron/test.ts` in `node-core-integration-tests` was still
timing out occasionally on CI (issue #20971). The scenario exits the
child process after `5000` ms and the vitest test uses the default
timeout, leaving little headroom for two cron ticks (`* * * * * *`) plus
the HTTP envelope round-trips through `withMockSentryServer()`.

## Root cause

The sibling test in `node-integration-tests` had the same flake symptom
previously (#20652 / #20663) and was already adjusted to a `15_000` ms
process-exit timeout and a `30_000` ms vitest test timeout. The
`node-core-integration-tests` version still carried the original tight
budgets (`5000` ms / vitest default), so it remained susceptible to the
same timing pressure.

This change mirrors the working sibling pattern verbatim — no assertions
are altered, no coverage is loosened, only the wall-clock budget grows.

Fixes #20971

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>

Author: github-actions[bot]

dev-packages/node-core-integration-tests/suites/cron/cron/scenario.ts

@@ -28,4 +28,4 @@ cron.start();
 
 setTimeout(() => {
   process.exit();
-}, 5000);
+}, 15_000);

dev-packages/node-core-integration-tests/suites/cron/cron/test.ts

@@ -5,7 +5,7 @@ afterAll(() => {
   cleanupChildProcesses();
 });
 
-test('cron instrumentation', async () => {
+test('cron instrumentation', { timeout: 30_000 }, async () => {
   await createRunner(__dirname, 'scenario.ts')
     .withMockSentryServer()
     .expect({