fix: use JsonValue for models.JSONField to accept dict input

[yulinfeng000]

Summary

Fixes the json_type ValidationError raised when a ModelSchema built from a Django model containing JSONField is populated from a database row.

• convert_postgres_field_to_string and convert_field_to_json_string now produce pydantic.JsonValue instead of pydantic.Json.
• JsonValue accepts the Python dict/list values the Django ORM returns; Json required a serialized string/bytes payload, which is the source of the bug.
• Updated tests/test_v2_pydantic/test_converters.py::test_django_31_fields to assert the new (correct) behavior: dict input is now valid, and the JSON schema fragment is JsonValue's $ref form.

Related issue

Refs eadwinCode/django-ninja-extra#267

Note on Pydantic v1 compatibility

pydantic.JsonValue was introduced in Pydantic 2.5. This PR's top-level import will fail under Pydantic v1, which is still permitted by pyproject.toml (pydantic>=1.7.4,...,<3.0.0).

I'm happy to follow up with either:

1. Gate by version — keep Json (or plain dict) for Pydantic v1 behind an IS_PYDANTIC_V1 branch, OR
2. Drop v1 support — bump the floor to pydantic>=2.5 in pyproject.toml and remove the v1 branches.

Let me know which direction you prefer and I'll send a follow-up commit.

Test plan

• pytest tests/test_v2_pydantic/test_converters.py::test_django_31_fields passes
• ruff check ninja_schema tests clean on the modified files
• Manual: ModelSchema with a JSONField accepts {"any": "data"} directly without raising ValidationError
• pytest tests/test_v1_pydantic/... — see "Note on Pydantic v1" above