fix: generated mutation fields get query descriptions #398

StrawchemyField.description (src/strawchemy/schema/field.py:667-681) builds its "Fetch … from the X collection" template for every field. The mutation field classes in src/strawchemy/schema/mutation/fields.py don't override it, so generated mutations get query descriptions.

Evidence, from tests/unit/mapping/__snapshots__/test_schemas/test_query_and_mutations.gql:144:

"""Fetch object from the FruitType collection by id"""
createFruit(data: FruitInput!): FruitType!

test_plain_type_fields_accepted.gql shows the same for createColor, createColors and createPlain. Fields with a custom resolver that return a plain Strawberry type (e.g. plain: [Plain!]!) also get a "Fetch objects from the Plain collection" description, although Strawchemy fetches nothing for them.

Expected: mutation fields get a mutation-specific description (or none), and resolver fields returning a non-Strawchemy type get none.