Fix custom base model support

[JasperTey]

Changed

• Implement more robust handling of base models when generating a domain model with ddd:model:
  • If the configured ddd.base_model exists (evaluated using class_exists), base model generation is skipped.
  • If ddd.base_model does not exist and falls under a domain namespace, base model will be generated.
  • Falling under a domain namespace means Domain\**\Models\SomeBaseModel.
  • For example, if ddd.base_model were set to App\Models\CustomAppBaseModel or Illuminate\Database\Eloquent\NonExistentModel, they fall outside of the domain namespace and won't be generated on your behalf.

Fixed

• Resolve long-standing issue where ddd:model would not properly detect whether the configured ddd.base_model already exists, leading to unpredictable results when ddd.base_model deviated from the default Domain\Shared\Models\BaseModel.