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-object-metadata/utils/from-delete-object-input-to-flat-field-metadatas-to-delete.util.ts

@@ -59,7 +59,11 @@ export const fromDeleteObjectInputToFlatFieldMetadatasToDelete = ({
     });
   const flatFieldMetadatasToDelete = objectFlatFieldMetadatas.flatMap(
     (flatFieldMetadata) => {
-      if (isMorphOrRelationFlatFieldMetadata(flatFieldMetadata)) {
+      if (
+        isMorphOrRelationFlatFieldMetadata(flatFieldMetadata) &&
+        flatFieldMetadata.relationTargetObjectMetadataId !==
+          objectMetadataToDeleteId
+      ) {
         const relationTargetFlatFieldMetadata =
           findRelationFlatFieldMetadataTargetFlatFieldMetadataOrThrow({
             flatFieldMetadata,