fix: validate registration invite before password validation

[10done]

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Changes

Fixes #7695
Moved _validate_registration_invite() to execute before super().validate()
in CustomUserCreateSerializer.validate().

Uninvited users should be rejected immediately with 403 before the server
performs any further validation. The previous ordering also had a minor
security implication — it leaked password rule feedback to users who were
not authorised to register at all.

How did you test this code?

• test_register__without_invite_when_disabled__returns_forbidden — passes
  deterministically
• Full custom_auth unit + integration test suite — all passing

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (cbcac64) to head (529d57f).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7711      +/-   ##
==========================================
- Coverage   98.52%   98.37%   -0.16%     
==========================================
  Files        1444     1444              
  Lines       54971    54968       -3     
==========================================
- Hits        54161    54074      -87     
- Misses        810      894      +84     

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[matthewelwell]

/gemini review

[gemini-code-assist]

Code Review

This pull request moves the registration invite validation to the beginning of the validate method in api/custom_auth/serializers.py. However, this change exposes a user enumeration vulnerability because Django Rest Framework's field-level validators run before the serializer's validate method. This allows uninvited users to determine if an email is registered based on whether they receive a 400 (email exists) or a 403 (no invite) response. It is recommended to remove the default UniqueValidator and perform the uniqueness check manually after the invite validation.