Fix dashboard new tab creation (#17971)

## Before


https://github.com/user-attachments/assets/cbc60013-8a5e-4af8-b02c-7dfbaf0c15e5


## After 


https://github.com/user-attachments/assets/1fd6f9f7-e774-4c63-aa80-50b33d2a4c59

packages/twenty-server/src/engine/metadata-modules/page-layout/services/page-layout-update.service.ts

@@ -7,6 +7,7 @@ import { v4 } from 'uuid';
 import { ApplicationService } from 'src/engine/core-modules/application/services/application.service';
 import { WorkspaceManyOrAllFlatEntityMapsCacheService } from 'src/engine/metadata-modules/flat-entity/services/workspace-many-or-all-flat-entity-maps-cache.service';
 import { AllFlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/types/all-flat-entity-maps.type';
+import { addFlatEntityToFlatEntityMapsOrThrow } from 'src/engine/metadata-modules/flat-entity/utils/add-flat-entity-to-flat-entity-maps-or-throw.util';
 import { findFlatEntityByIdInFlatEntityMapsOrThrow } from 'src/engine/metadata-modules/flat-entity/utils/find-flat-entity-by-id-in-flat-entity-maps-or-throw.util';
 import { findFlatEntityByIdInFlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/utils/find-flat-entity-by-id-in-flat-entity-maps.util';
 import { resolveEntityRelationUniversalIdentifiers } from 'src/engine/metadata-modules/flat-entity/utils/resolve-entity-relation-universal-identifiers.util';
@@ -123,11 +124,20 @@ export class PageLayoutUpdateService {
         },
       );
 
+    const optimisticFlatPageLayoutTabMaps = tabsToCreate.reduce(
+      (maps, tab) =>
+        addFlatEntityToFlatEntityMapsOrThrow({
+          flatEntity: tab,
+          flatEntityMaps: maps,
+        }),
+      flatPageLayoutTabMaps,
+    );
+
     const { widgetsToCreate, widgetsToUpdate, widgetsToDelete } =
       this.computeWidgetOperationsForAllTabs({
         tabs,
         flatPageLayoutWidgetMaps,
-        flatPageLayoutTabMaps,
+        flatPageLayoutTabMaps: optimisticFlatPageLayoutTabMaps,
         flatObjectMetadataMaps,
         workspaceId,
         workspaceCustomApplicationId: workspaceCustomFlatApplication.id,