pydantic_v2 base class does not use the RootModel

[amarion35]

Describe the bug
I want to generate models from this schema https://vega.github.io/schema/vega-lite/v5.json. I want to use pydantic v2 but the generated code is incorrect.

Firstly, the BaseModel is imported like this : from pydantic_v2 import BaseModel
Secondly, it uses the BaseModel but I expect it to use the RootModel.

To Reproduce

Example schema:
https://vega.github.io/schema/vega-lite/v5.json

Used commandline:

datamodel-codegen \
    --input-file-type jsonschema \
    --input v5.json \
    --output ./vegalite_spec.py \
    --target-python-version 3.11 \
    --base-class pydantic_v2.BaseModel \
    --use-standard-collections \
    --use-union-operator \
    --use-double-quotes

Observed behavior

# generated by datamodel-codegen:
#   filename:  v5.json
#   timestamp: 2024-12-18T16:56:18+00:00

from __future__ import annotations

from enum import Enum
from typing import Any

from pydantic import AnyUrl, Extra, Field, confloat, constr
from pydantic_v2 import BaseModel
...

Expected behavior
The import statements should be a valid import statement.
It should use RootModel instead of BaseModel.

# generated by datamodel-codegen:
#   filename:  v5.json
#   timestamp: 2024-09-30T08:54:36+00:00

from __future__ import annotations

from enum import Enum
from typing import Any, Literal

from pydantic import AnyUrl, BaseModel, ConfigDict, Field, RootModel
...

Version:

• OS: Ubuntu 20.04
• Python version: 3.11.6
• datamodel-code-generator version: 0.26.4

Additional context
I managed to generate the correct model a few month ago but I lost the exact command I used. So I have the expected code.

Related Issues
#2201
#1435