fix: add null validation for environment in FeatureStateSerializerBasic

[Pwongchaiya]

Summary

• Added validation in FeatureStateSerializerBasic.validate_environment to raise a ValidationError when the environment is None, preventing a 500 error response.

Test plan

• Verify that sending a request with a null environment returns a 400 with a descriptive error message instead of a 500.
• Verify existing environment validation (cannot change environment of a feature state) still works correctly.

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Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.26%. Comparing base (0d68a05) to head (01607d6).
⚠️ Report is 82 commits behind head on main.

Files with missing lines	Patch %	Lines
api/features/serializers.py	50.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (100.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7099      +/-   ##
==========================================
- Coverage   98.31%   98.26%   -0.06%     
==========================================
  Files        1335     1335              
  Lines       49760    49818      +58     
==========================================
+ Hits        48923    48952      +29     
- Misses        837      866      +29     

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[gagantrivedi]

Thanks for the fix — this is a real bug and worth addressing! A couple of suggestions:

Consider overriding the field instead of adding a validator check

The environment FK on the model has null=True, so DRF auto-generates the serializer field with allow_null=True. Since this serializer always expects a real environment (the validate() method falls back to self.context["environment"]), you could disallow null at the field level instead:

class FeatureStateSerializerBasic(WritableNestedModelSerializer):
    environment = serializers.PrimaryKeyRelatedField(
        queryset=Environment.objects.all(),
        allow_null=False,
    )

This way DRF handles the null rejection automatically with a standard error message, and validate_environment only needs to deal with the "cannot change environment" logic.

Tests needed

The project requires 100% diff coverage for backend PRs, so this will need at least one test — e.g. sending "environment": null and asserting a 400 response.