test(sveltekit): Unflake SvelteKit fetch proxy E2E test (#15311)

Ran both tests 100x in CI to check if the flakes are gone
([1](https://github.com/getsentry/sentry-javascript/actions/runs/13160819942/job/36729143461?pr=15311#step:14:80),
[2](https://github.com/getsentry/sentry-javascript/actions/runs/13160819942/job/36729144129?pr=15311#step:14:82)).
Seems so! The reason for the flake was that we didn't wait long enough
until the Sentry SDK was intiialized before checking for it.

closes #15269

dev-packages/e2e-tests/test-applications/sveltekit-2.5.0-twp/tests/sdk.test.ts

@@ -4,12 +4,10 @@ test.describe('SDK-internal behavior', () => {
   test('Injects fetch proxy script for SvelteKit<2.16.0', async ({ page }) => {
     await page.goto('/');
 
-    const sentryCarrier = await page.evaluate('typeof window.__SENTRY__');
+    // @ts-expect-error this is defined
+    await page.waitForFunction(() => typeof window.__SENTRY__ === 'object');
     const proxyHandle = await page.evaluate('typeof window._sentryFetchProxy');
 
-    // sanity check
-    expect(sentryCarrier).toBe('object');
-
     // fetch proxy script ran
     expect(proxyHandle).toBe('function');
   });

dev-packages/e2e-tests/test-applications/sveltekit-2/tests/sdk.test.ts

@@ -4,11 +4,11 @@ import { waitForInitialPageload } from './utils';
 test.describe('SDK-internal behavior', () => {
   test("Doesn't inject fetch proxy script for SvelteKit>=2.16.0", async ({ page }) => {
     await waitForInitialPageload(page, { route: '/' });
-    const sentryCarrier = await page.evaluate('typeof window.__SENTRY__');
-    const proxyHandle = await page.evaluate('typeof window._sentryFetchProxy');
 
-    // sanity check
-    expect(sentryCarrier).toBe('object');
+    // @ts-expect-error this is defined
+    await page.waitForFunction(() => typeof window.__SENTRY__ === 'object');
+
+    const proxyHandle = await page.evaluate('typeof window._sentryFetchProxy');
 
     // fetch proxy script didn't run
     expect(proxyHandle).toBe('undefined');