test(browser): Wait for the LCP handlers to see the image before finalizing

On Linux, Chromium reports the body as a first LCP candidate before the
image. The subject's handlers get the candidates through an observer of
their own, and finalizing LCP with the click while only the body was
delivered to them froze that value, so the test failed on every CI run of
some trees while passing locally.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: chargome

dev-packages/browser-integration-tests/suites/tracing/metrics/handlers-lcp/test.ts

@@ -28,6 +28,9 @@ sentryTest(
     const lcpPromise = waitForStreamedSpan(page, span => getSpanOp(span) === 'ui.webvital.lcp');
     await page.goto(url);
     await waitForLcpCandidate(page, 'img');
+    // The handlers observe LCP through their own observer; on Linux, Chromium reports the body before
+    // the image, and finalizing LCP while only the body has been delivered to them freezes that value.
+    await page.waitForFunction('window._LCP === 107400 && window._LCP2 === 107400');
     await page.locator('button').click();
     const lcpSpan = await lcpPromise;