POST, PUT and PATCH for statements do not require the "type" field to be provided in the request. It therefore needs to be added to the serialisation for internal use:

• in a separate patch, first rename Wikibase\Repo\RestApi\Domain\Serializers to Wikibase\Repo\RestApi\Domain\Serialization
• inside that namespace, create a wrapper around the existing StatementDeserializer, which adds the 'type' => 'statement' field
• use the newly created wrapped Deserializer in JsonDiffStatementPatcher and SnakValidatorStatementValidator
• remove type fields from all statement serializations in unit, e2e and schema tests