Partially updating a multivariate option returns 500

[khvn26]

PATCH /api/v1/projects/{project}/features/{feature}/mv-options/{id}/ returns a 500 when the payload leaves out default_percentage_allocation, which prevents updates to change only a variant key.

Steps To Reproduce

1. Create a multivariate feature with at least one variant.
2. PATCH .../mv-options/{id}/ with {"feature": <feature_id>, "key": "hero"}.
3. The response is 500.

Expected behaviour

A partial update succeeds, leaving the allocation as it is.