Fix versionedNotulen never fetching filecontent

[abeforgit]

This pr does 2 related things:
The first commit is the actual bugfix: we were checking for the existence of a file in the wrong way, causing it to always generate a false negative, creating a new file instead of using the old one.

Then a different part of the code was getting the content, and it only allowed there to be a single file. This is a good check, but the error it generated dissapeared and never made it to the frontend, so we couldn't handle it if we tried.

For this I adjusted our usage of Tasks to be compliant with the model

needs auth config from lblod/app-gelinkt-notuleren#213

jira: GN-5226

[elpoelma]

The re-use of the notulen file seems to work well now for the second signature, confirmed by debugging :)
I did not test the error producing/handling yet, any tips on how to do this best?

I did notice an additional error (unrelated to this PR): lezer mock-users are not able to provide the first signature, this is not expected right?
Update: this is probably caused due to the inability to make files as a 'lezer'. Only accounts with the 'Schrijver' role may create files (https://github.com/lblod/app-gelinkt-notuleren/blob/587962bf82a7cd5c47ccafd7826fb37405b03680/config/authorization/config.ex#L154C26-L154C97)