Fix race conditions in our template tests

[FranjoMindek]

Currently, our templates e2e tests can fail due to race conditions.
Once such race condition is around updating user's subscription status in time before we read and test the user's subscription status.

This one was recorded by me:

1. invoice.paid webhook event sent timestamp - 2:24:58

2. Backend reacting to the invoice.paid event and updating the subscription status: 2:25:07

3. Client reading the user's subscription status on the Account page: 2:25:05

This results in us failing the e2e-tests.

[cprecioso]

Link to failing CI run: https://github.com/wasp-lang/open-saas/actions/runs/21027744847/job/60456195514?pr=600

[wang2-lat]

Hi @FranjoMindek 👋

I've submitted a fix for this race condition issue.

The problem: Tests were checking subscription status before the webhook finished processing.

The solution: Added a polling mechanism that waits up to 10 seconds for the subscription status to update, retrying every 500ms.

PR: #628

This should resolve the flaky test failures you reported in the CI run.

[wang2-lat]

可以在测试中加个轮询重试机制，比如用 waitFor 或 retry 函数在读取订阅状态前等待几秒并重试，直到状态更新完成或超时。另外也可以考虑在 webhook 处理完成后发个内部事件，测试监听这个事件再进行断言。