Inconsistent value type when validating nested schema fields

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**Problem:**

When a Nested schema has a validation error its `@post_load` hooks are not invoked bypassing any transformations done there. This means the parent schema’s `@validates` and `@validate_schema` hooks have to deal with both the transformed and untransformed child values.

Originally I encountered this using `marshmallow-sqlalchemy` with `load_instance=True`.

**Steps to reproduce:**

The code below shows the problem.

```python
import dataclasses
import uuid

import marshmallow as ma
import pytest

@dataclasses.dataclass
class ChildModel:
uuid: uuid.UUID
name: str

class Child(ma.Schema):
uuid = ma.fields.UUID(required=True)
name = ma.fields.String(required=True)

@ma.post_load
def transform(self, data, **_kwargs):
return ChildModel(**data)

class Parent(ma.Schema):
child = ma.fields.Nested("Child")

@ma.validates("child")
def validate_child(self, value, **_kwargs):
if value:
# This fails when the child has a validation error as then its @post_load is not called.
assert isinstance(value, ChildModel)

def test_load_valid_uuid():
data = {"child": {"uuid": "c81505b4-258b-4912-b8bc-8ac913d56736", "name": "test"}}
result = Parent().load(data)
assert isinstance(result["child"], ChildModel)

def test_load_invalid_uuid():
data = {"child": {"uuid": "invalid-uuid", "name": "test"}}
with pytest.raises(ma.exceptions.ValidationError):
result = Parent().load(data)
```

When the child schema validates then `Parent.validate_child` is called with a ChildModel instance, but if the child data is invalid it gets called with a dict instance instead.

Start with the Nested schema loading and validation flow represented by Parent, Child, and the @post_load and @validates hooks in the reproduction. Run the supplied pytest cases and trace why post_load is skipped for invalid child data. Done means parent validation sees a consistent child value type in both valid and invalid nested-data paths, with regression coverage for the reproduction.

Written by the indexing model from the issue text.