fix(api): Allow deactivation of relation fields (#13202)

_(AI generated)_

### Summary

This PR fixes a validation bug in the GraphQL API that prevented
relation fields from being programmatically deactivated. The validation
was incorrectly triggering a "name cannot be changed" error even when
the update payload did not include a name, making it impossible to
disable the field.

Issue #13200 

### Problem

- The [updateOneField] mutation failed when trying to set `isActive:
false` on a `RELATION` field.
- The root cause was a validation check in
[FieldMetadataValidationService] that compared the incoming `name` with
the existing one. If the input `name` was `undefined`, the check
`undefined !== existingName` would incorrectly fail.
- This created a catch-22 where a field could not be deleted (because it
had to be deactivated first) and could not be deactivated (due to this
validation error).

### Solution

- The validation logic in [field-metadata-validation.service.ts] has
been updated to only check for a name change if a new name is
**explicitly provided** in the input
(`isDefined(fieldMetadataInput.name)`).
- This change correctly enforces the rule that relation field names
cannot be changed, while allowing other properties like `isActive` to be
updated without issue.

### How to Test

1.  Create a custom field of type `RELATION`.
2. Using the GraphQL API, call the [updateOneField] mutation with the
field's ID and the payload `{ "isActive": false }`.
3.  Verify that the mutation succeeds and the field is now inactive.
4.  Call the [deleteOneField] mutation to delete the field.
5.  Verify that the deletion is successful.

### Additional Changes

_to be deleted if not necessary_

- Added a new integration test
[successful-field-metadata-relation-update.integration-spec.ts] to cover
this specific use case and prevent future regressions. The existing test
for failing updates remains untouched and continues to pass.

packages/twenty-server/src/engine/metadata-modules/field-metadata/services/field-metadata-validation.service.ts

@@ -195,6 +195,7 @@ export class FieldMetadataValidationService {
     if (
       isRelationField &&
       isDefined(existingFieldMetadata) &&
+      isDefined(fieldMetadataInput.name) &&
       fieldMetadataInput.name !== existingFieldMetadata.name
     ) {
       throw new FieldMetadataException(