refactor: rename ObjectMetadataItem to EnrichedObjectMetadataItem and clean up metadata flows (#18830)

## Summary

- Renames `ObjectMetadataItem` to `EnrichedObjectMetadataItem` across
the entire frontend (~440 files) to clarify that this type includes
derived fields (`readableFields`, `updatableFields`, nested `fields[]`,
`indexMetadatas[]`) computed at read time from the metadata store
- Creates `splitObjectMetadataGqlResponse` that goes directly from a
GraphQL `ObjectMetadataItemsQuery` response to flat store items
(combining the old
`mapPaginatedObjectMetadataItemsToObjectMetadataItems` +
`splitObjectMetadataItemWithRelated` two-step flow into one call)
- Removes `ObjectMetadataItemWithRelated` type and all "WithRelated"
naming
- Renames `generatedMockObjectMetadataItems` to
`generateTestEnrichedObjectMetadataItemsMock` to make it clear this is
test-only enriched data
- Deletes `useLoadMockedObjectMetadataItems` hook (consolidated into
`useLoadMockedMinimalMetadata`)
- Ensures nothing destined for the metadata store computes
`readableFields`/`updatableFields` (preventing the localStorage bloat
from #18809)

## Type hierarchy (before → after)

**Before:**
```
ObjectMetadataItemsQuery → mapPaginated → ObjectMetadataItemWithRelated → enrich → ObjectMetadataItem
                                        → split → FlatObjectMetadataItem (store)
```

**After:**
```
ObjectMetadataItemsQuery → splitObjectMetadataGqlResponse → FlatObjectMetadataItem (store)
                         → mapPaginated + enrich (tests only) → EnrichedObjectMetadataItem
```

## Test plan

- [x] `npx nx typecheck twenty-front` passes
- [x] `npx nx test twenty-front` passes (767 suites, 4505 tests)
- [x] `npx nx lint twenty-front` passes
- [ ] CI checks pass


Made with [Cursor](https://cursor.com)

packages/twenty-front/src/modules/activities/utils/getActivityTargetFieldNameForObject.ts

@@ -1,6 +1,6 @@
 import { getActivityTargetObjectFieldIdName } from '@/activities/utils/getActivityTargetObjectFieldIdName';
 import { CoreObjectNameSingular } from 'twenty-shared/types';
-import { type ObjectMetadataItem } from '@/object-metadata/types/ObjectMetadataItem';
+import { type EnrichedObjectMetadataItem } from '@/object-metadata/types/EnrichedObjectMetadataItem';
 import { isDefined } from 'twenty-shared/utils';
 
 type GetActivityTargetFieldNameForObjectArgs = {
@@ -8,7 +8,7 @@ type GetActivityTargetFieldNameForObjectArgs = {
     | CoreObjectNameSingular.Note
     | CoreObjectNameSingular.Task;
   targetObjectMetadataId: string;
-  objectMetadataItems: ObjectMetadataItem[];
+  objectMetadataItems: EnrichedObjectMetadataItem[];
   isMorphRelation?: boolean;
 };