Fix fields widget new field visibility (#21111)

Fixes https://github.com/twentyhq/twenty/issues/21043

## Context
Newly created fields were never added to FIELDS widgets, regardless of
the "Set fields created in the future as visible" toggle. The widget's
newFieldDefaultVisibility was null on widgets that never explicitly set
it (it was never populated at creation), so the backend skipped them and
no view field was created.

## Implementation
Keep newFieldDefaultVisibility nullable with false (not visible) as the
behavior when not provided.
The FE now reflects that properly and shows "un-toggled" when it's null
(iso with BE behavior).

The fix also ensures the value is explicitly set to true wherever it
should be:
- Set newFieldDefaultVisibility: true at every FIELDS widget creation
path (backend default record-page layout, frontend
createDefaultFieldsWidget + useTemporaryFieldsConfiguration);
- Added a 2-9 workspace upgrade command that backfills true onto
existing standard FIELDS widgets where the value is null.

packages/twenty-server/src/engine/metadata-modules/flat-page-layout-widget/validators/utils/validate-fields-flat-page-layout-widget-for-creation.util.ts

@@ -40,6 +40,25 @@ export const validateFieldsFlatPageLayoutWidgetForCreation = (
     return errors;
   }
 
+  const newFieldDefaultVisibility = (
+    universalConfiguration as {
+      configurationType: string;
+      newFieldDefaultVisibility?: unknown;
+    }
+  ).newFieldDefaultVisibility;
+
+  if (
+    isDefined(newFieldDefaultVisibility) &&
+    typeof newFieldDefaultVisibility !== 'boolean'
+  ) {
+    errors.push({
+      code: PageLayoutWidgetExceptionCode.INVALID_PAGE_LAYOUT_WIDGET_DATA,
+      message: t`Invalid newFieldDefaultVisibility for fields widget "${widgetTitle}". Expected a boolean`,
+      userFriendlyMessage: msg`Invalid newFieldDefaultVisibility for fields widget`,
+      value: newFieldDefaultVisibility,
+    });
+  }
+
   const viewUniversalIdentifier = (
     universalConfiguration as {
       configurationType: string;

packages/twenty-server/src/engine/metadata-modules/object-metadata/utils/compute-flat-default-record-page-layout-to-create.util.ts

@@ -78,6 +78,7 @@ export const computeFlatDefaultRecordPageLayoutToCreate = ({
       ? {
           configurationType: WidgetConfigurationType.FIELDS,
           viewId: recordPageFieldsView.id,
+          newFieldDefaultVisibility: true,
         }
       : {
           configurationType:
@@ -90,6 +91,7 @@ export const computeFlatDefaultRecordPageLayoutToCreate = ({
       ? {
           configurationType: WidgetConfigurationType.FIELDS,
           viewUniversalIdentifier: recordPageFieldsView.universalIdentifier,
+          newFieldDefaultVisibility: true,
         }
       : {
           configurationType: