fix: check-split-length-before-accessing-with-index

[Zaimwa9]

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• I have added information to docs/ if required so people know about the feature!
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closes #4951

Changes

• Check split length before accessing

How did you test this code?

Added test in models
To reproduce:
Try login with invalid email

New error (without auth-controller cf this PR):

    "non_field_errors": [
        "Unable to log in with provided credentials."
    ]
}

[vercel]

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3 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored	Preview		Aug 13, 2025 3:05pm
flagsmith-frontend-preview	⬜️ Ignored	Preview		Aug 13, 2025 3:05pm
flagsmith-frontend-staging	⬜️ Ignored	Preview		Aug 13, 2025 3:05pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-e2e:pr-5848	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api-test:pr-5848	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-5848	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-5848	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-5848	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-5848	Finished ✅	Results ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.84%. Comparing base (1411f54) to head (2f0368e).
⚠️ Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5848      +/-   ##
==========================================
+ Coverage   97.82%   97.84%   +0.02%     
==========================================
  Files        1258     1261       +3     
  Lines       44745    44899     +154     
==========================================
+ Hits        43771    43933     +162     
+ Misses        974      966       -8     

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