Contributes to #346563

**Problem**

`ForkService` class returns a Project object (correct and saved or
invalid and nonpersisted). This flow doesn't match a service class
expectation to return a `ServiceResponse` method.

**Solution**

* Migrate `ForkService#execute` to return a `ServiceResponse` object.
* Update code that uses ForkService to accept a new return type

Changelog: other