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 75.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 50.33%. Comparing base (f475aea) to head (70c6cb3).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
x/http.go	60.00%	2 Missing ⚠️
selfservice/flow/login/handler.go	66.66%	1 Missing ⚠️
selfservice/flow/login/hook.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4336      +/-   ##
==========================================
- Coverage   50.33%   50.33%   -0.01%     
==========================================
  Files         372      372              
  Lines       27424    27428       +4     
==========================================
+ Hits        13804    13806       +2     
- Misses      12014    12016       +2     
  Partials     1606     1606              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.