[SPEC CHANGE] Required fields in `x_mitre_related_assets`

[seansica]

Make name, description, and related_asset_sectors required — presently, only name is required:

export const relatedAssetSchema = z
  .object({
    name: nameSchema.meta({
      description: 'Sector-specific name or alias for the related asset',
    }),

    related_asset_sectors: xMitreSectorsSchema.optional(), // <-- remove optional
    description: descriptionSchema
        .optional() // <-- remove optional
        .meta({
          description: 'How the related asset connects to the primary asset definition',
      }),
  })
  .meta({
    description: 'The `related_asset` object provides sector-specific asset variations and aliases',
  });