fix: registration should accept hydra login

[Benehiko]

Related issue(s)

https://github.com/ory-corp/cloud/issues/5493

Using propt=registration on an OAuth2 Hydra login flow, the request is routed to the Kratos registration handler. This handler immediately redirects to the logout flow when there is an already existing session, which fails and ends up at the base /welcome page on the Account Experience (AX).

This PR adds the functionality of accepting the login request in the registration handler if the user has a session and if the hydra skip=true is in the login request.

When the skip=false is set, the request is redirected to the login flow with the login_challenge, which will complete the flow normally.

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[hperl]

Thanks for looking into this! The approach is sensible, I just think we should look for prompt=login instead of skip.

[codecov]

Codecov Report

Merging #3592 (0bed0c9) into master (fdf4956) will decrease coverage by 0.03%.
Report is 1 commits behind head on master.
The diff coverage is 66.00%.

❗ Current head 0bed0c9 differs from pull request most recent head 565e4c6. Consider uploading reports for the commit 565e4c6 to get more accurate results

@@            Coverage Diff             @@
##           master    #3592      +/-   ##
==========================================
- Coverage   78.24%   78.22%   -0.03%     
==========================================
  Files         341      341              
  Lines       23098    23129      +31     
==========================================
+ Hits        18073    18092      +19     
- Misses       3674     3683       +9     
- Partials     1351     1354       +3     

Files	Coverage Δ
hydra/fake.go	75.00% <100.00%> (+2.27%)	⬆️
selfservice/flow/registration/handler.go	71.70% <63.04%> (-4.43%)	⬇️

... and 1 file with indirect coverage changes

[hperl]

LGTM!