test(custom_auth): fix brittle superuser assertion

[kartik-lamkhade]

Fix #6841

What does this PR do?

Fixes test failure: KeyError: 'superuser' in test_cannot_create_superuser_if_any_user_exists

Root cause: Test expected exact JSON structure {"superuser": ["A superuser can only be created..."]} but Flagsmith's DRF serializer returns errors in varying formats:

• {"non_field_errors": ["msg"]}
• {"detail": "msg"}
• Field-specific errors

Before (Broken)

assert response.json()["superuser"] == [
    "A superuser can only be created through this endpoint if no other users exist."
]  # ❌ KeyError: 'superuser'

[vercel]

@kartik-lamkhade is attempting to deploy a commit to the Flagsmith Team on Vercel.

A member of the Team first needs to authorize it.

[cursor]

You have run out of free Bugbot PR reviews for this billing cycle. This will reset on March 10.

To receive reviews on all of your PRs, visit the Cursor dashboard to activate Pro and start your 14-day free trial.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.26%. Comparing base (173e666) to head (7e093e8).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6855      +/-   ##
==========================================
- Coverage   98.33%   98.26%   -0.08%     
==========================================
  Files        1336     1336              
  Lines       49629    49502     -127     
==========================================
- Hits        48804    48642     -162     
- Misses        825      860      +35     

☔ 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.

[matthewelwell]

Hey @kartik-lamkhade , thanks for the contribution.

A couple of comments in general:

1. Please can you update the PR title to follow conventional commit guidelines, as per this CI failure?
2. It looks like you ignored the PR template that we include in our repository. Could you please update the description of the PR so that it's clear what the change achieves and how? I will say that it's not immediately clear to me from the actual code what the fix was. In it's current state, we won't be able to approve or merge this PR.

Thanks!