Field metadata relation edge cases exceptions coverage (#12866)

# Introduction
Following https://github.com/twentyhq/twenty/pull/12852

Discovered that:
- `relationCreationPayload` does not seem to be validated through the
input decorators
```ts
  // TODO @prastoin implement validation for this with validate nested and dedicated class instance
  @IsOptional()
  @Field(() => GraphQLJSON, { nullable: true })
  relationCreationPayload?: {
    targetObjectMetadataId: string;
    targetFieldLabel: string;
    targetFieldIcon: string;
    type: RelationType;
  };
```
- Sending an unknown `targetObjectMetadataId` generates an
`internal_server_error` `500` @guillim on the go
## Coverage
```ts
 PASS  test/integration/metadata/suites/object-metadata/failing-field-metadata-relation-creation.integration-spec.ts
  Field metadata relation creation should fail
    ✓ relation when targetFieldLabel is empty (109 ms)
    ✓ relation when targetFieldLabel exceeds maximum length (100 ms)
    ✓ relation when targetObjectMetadataId is unknown (97 ms)
    ✓ relation when targetFieldLabel contains only whitespace (103 ms)
    ✓ relation when targetFieldLabel conflicts with an existing field on target object metadata id (108 ms)

Test Suites: 1 passed, 1 total
Tests:       5 passed, 5 total
Snapshots:   5 passed, 5 total
Time:        2.629 s, estimated 3 s
```

packages/twenty-server/src/engine/metadata-modules/field-metadata/dtos/create-field.input.ts

@@ -22,6 +22,7 @@ export class CreateFieldInput extends OmitType(
   @IsOptional()
   isRemoteCreation?: boolean;
 
+  // TODO @prastoin implement validation for this with validate nested and dedicated class instance
   @IsOptional()
   @Field(() => GraphQLJSON, { nullable: true })
   relationCreationPayload?: {