fix(#5272): don't make Axios parse the content of the logfile as JSON

[cdprete]

Fixes #5272

[cdprete]

Sorry @SteKoe if I didn't write a test for this (to avoid that in the future it breaks again due to something else and it goes unnoticed).

I tried to use https://vitest.dev/guide/mocking/requests.html in the test directly instead of in the global setup (to not impact the other tests) to mock the request, but I didn't manage to find a way to make axios point with its baseURL to the URL I set in the mock server request handler.
Even by making the Instance object expose the axios configuration/setup, so that I could change it in the tests to point to my URL, it was anyway always pointing to something else.

I don't know if there is maybe an env variable or so to influence the behaviour.

[cdprete]

Ok, I should have dig deeper in the codebase 🤦

You already have it under https://github.com/codecentric/spring-boot-admin/blob/master/spring-boot-admin-server-ui/src/main/frontend/tests/setup.ts and use it.
Moreover, I could overwrite the handlers with https://mswjs.io/docs/best-practices/network-behavior-overrides.