Update API specifications with fern api update

[github-actions]

Update API specifications by running fern api update.

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Note

Low Risk
OpenAPI-only constraint change; runtime behavior is unchanged unless clients or codegen relied on accepting empty-string parentId values.

Overview
Regenerates openapi.json via Fern and tightens validation on parentId for ModelObject, ModelObjectRequired, and ModelObjectPartial.

Each of those schemas now includes minLength: 1 on parentId while it stays nullable: true, so null is still allowed for the root parent, but an empty string is no longer a valid parent ID when a string is sent.

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