handle parsing exception for OIDCHttpClient::checkOIDCIsAvailable()

[florentos17]

here is a fix for a bug pointed out by this user experience review.

• upon providing https://api.fastmail.com/jmap/session, the actual error is a 401 Unauthorized and this case was handeled in this pull request ;
• upon providing the correct https://api.fastmail.com, another error occured because the server answers with HTML instead of JSON, eg, FormatException. the correct behaviour, which i fix with the current PR, is to ignore webfinger and fall back to the classic credential form.

[github-actions]

This PR has been deployed to https://linagora.github.io/tmail-flutter/3459.

[chibenwa]

Please record a little video accessing a fastmail account after this change, if possible.

[tddang-linagora]

From the same discussion:

Our JMAP endpoint does not support basic auth

There's no point showing our own log in form. We need to add OAuth support in order for FastMail to work on Twake Mail.

[florentos17]

There's no point showing our own log in form. We need to add OAuth support in order for FastMail to work on Twake Mail.

correct, issue 3455 has already been open on the topic

Please record a little video accessing a fastmail account after this change, if possible.

thus with the current work we still cannot access a fastmail account, but the parsing exception is handled and the app falls back to the basic auth form instead of throwing an error:

fix-parsing-exception.mp4

[florentos17]

Is it ok if I force push here to clean the 39 commits that don't belong to this PR ?

[hoangdat]

first, please use git rebase to only pick your modification for this issue, then force push.

[florentos17]

@hoangdat @tddang-linagora @dab246 I rebased on latest master and checked that the demo is still the same, it is all good.

Tell me if you have any last comment

[dab246]

lgtm