fix: root_aggregations on a plain strawberry type fails at query time instead of definition

[gazorby]

Found while reviewing #389.

StrawchemyField._validate_type (src/strawchemy/schema/field.py) raises StrawchemyFieldError when root_aggregations is enabled on a field whose type is not a root aggregation type, but only for strawchemy types: a plain strawberry type is skipped. A root field without resolver then builds a repository at execution, and dto_model_from_type fails with AttributeError on __dto_model__.

Reproduction

@strawberry.type
class Plain:
    name: str

@strawberry.type
class Query:
    plain: list[Plain] = strawchemy.field(root_aggregations=True)

Schema builds; querying plain raises AttributeError instead of a StrawchemyFieldError at definition.

Expected

root_aggregations on a field whose type is not a strawchemy root aggregation type raises StrawchemyFieldError at definition, plain strawberry types included (at least for root fields without a custom resolver).