Replace calls to external API in e2e tests with something hosted locally on CI

### Description

We're currently calling `GET https://jsonplaceholder.typicode.com/todos/1` and others during e2e tests.
We've just seen a flake where this caused e2e tests to fail.
We should host something on CI for testing locally to not have flaky tests. Similar to the mock Sentry server.

For samples I'd recommend keeping what we currently have and only override for e2e test execution so customers who want to try the sample don't have to spin up another service.

先找出呼叫 jsonplaceholder.typicode.com 的 e2e 測試設定，並了解測試所用的 mock Sentry server 是如何託管的。在保留範例目前外部 API 行為的同時，為 e2e 執行提供 CI 本機端的等效方案；完成的標準是這些測試不再依賴外部服務，也不會在該服務無法使用時失敗。

由索引模型根據 Issue 內容生成。