fix: normalize boolean values consistently

[jacopotediosi]

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What does this PR do and why is it necessary?

I found that in some parts of the codebase, boolean values were not being obtained/validated/converted checking against valid_boolean_trues. This could lead to interpreting the string false as True, for example when retrieved from config.yaml or from user-provided inputs.

This PR fixes all such occurrences.

How was it tested? How can it be tested by the reviewer?

Go to http://127.0.0.1:5000/reverse_proxy_test/?authenticated=false
Before this PR, it showed the authenticated version of the reverse proxy test page, even if the authenticated parameter was false.
After this PR, it correctly shows the unauthenticated version of the page.

Was any kind of genAI (ChatGPT, Copilot etc) involved in creating this PR? Which one and how?

No

Any background context you want to provide?

What are the relevant tickets if any?

Screenshots (if appropriate)

Further notes