Fix v2 self relation field creation (#15382)

# Introduction
Fixing self relation field creation in v2
- When computing related flat field to delete on object metadata
deletion that has self relation fields
- On self relation creation validation name availability not searching
for the relation target field of the current object field if it's the
field being validated

## Coverage
- added CUD integration testing on self relation fields

close https://github.com/twentyhq/twenty/issues/15153

packages/twenty-server/src/engine/metadata-modules/flat-field-metadata/validators/utils/validate-flat-field-metadata-name-availability.util.ts

@@ -79,13 +79,17 @@ export const validateFlatFieldMetadataNameAvailability = ({
       }
 
       const targetFlatFieldMetadata =
-        remainingFlatEntityMapsToValidate?.byId[
-          existingFlatFieldMetadata.relationTargetFieldMetadataId
-        ] ??
-        findFlatEntityByIdInFlatEntityMapsOrThrow({
-          flatEntityId: existingFlatFieldMetadata.relationTargetFieldMetadataId,
-          flatEntityMaps: flatFieldMetadataMaps,
-        });
+        flatFieldMetadata.id ===
+        existingFlatFieldMetadata.relationTargetFieldMetadataId
+          ? flatFieldMetadata
+          : (remainingFlatEntityMapsToValidate?.byId[
+              existingFlatFieldMetadata.relationTargetFieldMetadataId
+            ] ??
+            findFlatEntityByIdInFlatEntityMapsOrThrow({
+              flatEntityId:
+                existingFlatFieldMetadata.relationTargetFieldMetadataId,
+              flatEntityMaps: flatFieldMetadataMaps,
+            }));
 
       if (!isMorphOrRelationFlatFieldMetadata(targetFlatFieldMetadata)) {
         return false;