Distribute BlockInRequest<D> over D

Aligns BlockInRequest with BlockInNestedResponse so that, when D is a
union of item-type definitions, the type expands into per-D variants
instead of a single object whose internal fields carry the union. This
restores type-narrowing on `attributes`, `relationships`, and the
`__itemTypeId` discriminator when consumers pattern-match by item_type.

Author: stefanoverna

packages/cma-client/src/fieldTypes/single_block.ts

@@ -93,9 +93,9 @@ export type NewBlockInRequest<
  * Also, 'meta' can always be omitted
  */
 export type BlockInRequest<D extends ItemTypeDefinition = ItemTypeDefinition> =
-  | UnchangedBlockInRequest
-  | UpdatedBlockInRequest<D>
-  | NewBlockInRequest<D>;
+  D extends unknown
+    ? UnchangedBlockInRequest | UpdatedBlockInRequest<D> | NewBlockInRequest<D>
+    : never;
 
 export type BlockInNestedResponse<
   D extends ItemTypeDefinition = ItemTypeDefinition,