core/oauth2: don't set state in responses if not supplied

[alxndrsn]

I think this fixes a bug in OAuth2 / OpenID Connect (OIDC) implementation, as reported at getodk/central-backend#1135 (comment)

I guess this PR would ideally include tests which would fail without the code changes.

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[BeryJu]

Thanks for the PR! I'll have to do some reading but iirc as part of OIDC the state parameter is required? In that case we'd have to enforce it being set when the openid scope is set

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.31%. Comparing base (49cf10e) to head (be46401).
Report is 91 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9735      +/-   ##
==========================================
- Coverage   92.46%   92.31%   -0.15%     
==========================================
  Files         669      704      +35     
  Lines       32751    34404    +1653     
==========================================
+ Hits        30282    31760    +1478     
- Misses       2469     2644     +175     

Flag	Coverage Δ
e2e	49.19% <60.00%> (-1.44%)	⬇️
integration	25.36% <0.00%> (-0.63%)	⬇️
unit	89.82% <100.00%> (+0.06%)	⬆️

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[alxndrsn]

iirc as part of OIDC the state parameter is required?

I may be reading the wrong place, but https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest reads:

state
RECOMMENDED. Opaque value used to maintain state between the request and the callback. Typically, Cross-Site Request Forgery (CSRF, XSRF) mitigation is done by cryptographically binding the value of this parameter with a browser cookie.