_(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.
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