fix(multivariate): Handled missing default_percentage_allocation in option validation

[adityapradhan10]

Thanks for submitting a PR! Please check the boxes below:

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Closes #6615

Fixes a KeyError when creating or updating multivariate options via the API without specifying optional fields.

Root cause: The validate() method in MultivariateFeatureOptionSerializer accessed attrs["feature"] and attrs["default_percentage_allocation"] directly, causing crashes on partial updates or creates when these fields were omitted.

Fixes a KeyError when creating or updating multivariate options via the API when optional fields are omitted.

Root cause: MultivariateFeatureOptionSerializer.validate() used attrs["feature"] and attrs["default_percentage_allocation"], which raises when those keys are missing on partial updates or when creating via nested routes without sending them.

Fix:

• feature: Resolved with fallbacks: attrs.get("feature"), then for updates self.instance.feature, then for nested routes the feature from URL kwargs (feature_pk / project_pk). The serializer marks feature as not required via extra_kwargs so POSTs without feature (e.g. when the feature is in the path) still reach validate().
• default_percentage_allocation: Resolved with attrs.get(..., instance_default_allocation), where instance_default_allocation is the instance value when present, or 100 for creates or legacy rows with NULL. The chosen value is written back into attrs["default_percentage_allocation"] so validation and persistence stay in sync and legacy NULL is corrected on save.

Note: No documentation changes required - this is a bug fix for the existing API behaviour, not a new feature.

How did you test this code?

• test_create_mv_option__without_feature_in_payload__uses_feature_from_url – Create without feature in the body (feature from URL) → 201 and correct feature / allocation.
• test_create_mv_option__without_default_percentage_allocation__uses_default – Create without default_percentage_allocation → 201 and value 100.
• test_create_mv_option__without_default_percentage_allocation_with_existing_sibling_and_total_gt_100__returns_400 – Create without allocation when a sibling has 50% → 400 and "Invalid percentage allocation".
• test_partial_update_mv_option__without_feature_and_allocation__uses_existing_values – PATCH without feature or default_percentage_allocation → 200 and existing values kept.
• test_partial_update_mv_option__handles_legacy_null_default_percentage_allocation – Serializer update with an instance that has default_percentage_allocation is None in memory → validates as 100 and persists 100 (legacy NULL handling).
• test_partial_update_default_percentage_allocation__sibling_total_gt_100__returns_400 – PATCH that would push total allocation over 100% → 400 and "Invalid percentage allocation".

[vercel]

@adityapradhan10 is attempting to deploy a commit to the Flagsmith Team on Vercel.

A member of the Team first needs to authorize it.

[khvn26]

The fix looks solid to me. Need to address test naming convention/typing.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with Cloud Agents, enable Autofix in the Cursor dashboard.}