fix: incorrect response code on account linking

[aeneasr]

BREAKING CHANGES: Account linking incorrectly returned a 200 OK status code even though the login flow was not completed successfully. Going forward, the correct 400 OK status code will be sent when using the API flow or Accept: application/json.

Related issue(s)

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

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.51%. Comparing base (bf674d1) to head (7edfb84).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
selfservice/flow/login/hook.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4336      +/-   ##
==========================================
+ Coverage   78.49%   78.51%   +0.01%     
==========================================
  Files         366      366              
  Lines       27130    27134       +4     
==========================================
+ Hits        21295    21303       +8     
+ Misses       4207     4205       -2     
+ Partials     1628     1626       -2     

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