fix: strawchemy.field annotations cannot resolve plain strawberry types under postponed annotations #393

Found while working on #391.

Under from __future__ import annotations, a strawchemy.field() annotation that references a plain strawberry type never resolves, so the schema fails to build:

from __future__ import annotations

@strawberry.type
class Plain:
    name: str

@strawberry.type
class Query:
    color_or_plain: ColorType | Plain = strawchemy.field()

UnresolvedFieldTypeError: Could not resolve the type of 'color_or_plain'. The same happens with a custom ErrorType subclass in a mutation union.

The annotation is evaluated against the strawchemy registry namespace only (StrawchemyField.type setter using self.registry_namespace in src/strawchemy/schema/field.py, and _annotation_namespace() in src/strawchemy/mapper.py), which does not contain types defined outside strawchemy.

Expected

Annotations resolve plain strawberry types from the module where the field is declared, as they do without postponed annotations.