Enable defer_build for Entity.Model and Sealable.Model (#6867)

Enables lazy construction of validators and serializers by setting
`defer_build=True`. This can reduce import-time overhead and improve performance
in contexts where models are not always used.

Author: danielhollas

src/aiida/orm/entities.py

@@ -181,7 +181,7 @@ class Entity(abc.ABC, Generic[BackendEntityType, CollectionType], metaclass=Enti
     _CLS_COLLECTION: Type[CollectionType] = Collection  # type: ignore[assignment]
     _logger = log.AIIDA_LOGGER.getChild('orm.entities')
 
-    class Model(BaseModel):
+    class Model(BaseModel, defer_build=True):
         pk: Optional[int] = MetadataField(
             None,
             description='The primary key of the entity. Can be `None` if the entity is not yet stored.',

src/aiida/orm/utils/mixins.py

@@ -183,7 +183,7 @@ class Sealable:
 
     SEALED_KEY = 'sealed'
 
-    class Model(pydantic.BaseModel):
+    class Model(pydantic.BaseModel, defer_build=True):
         sealed: bool = MetadataField(description='Whether the node is sealed')
 
     @classproperty