Combination of '--collapse-root-models and --field-constraints` errors out

[ostefano]

Describe the bug
Combination of --collapse-root-models and --field-constraints errors out:

File "/path/.venv/lib/python3.13/site-packages/datamodel_code_generator/__main__.py", line 494, in main
    generate(
    ~~~~~~~~^
        input_=config.url or config.input or sys.stdin.read(),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<67 lines>...
        no_alias=config.no_alias,
        ^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/path/.venv/lib/python3.13/site-packages/datamodel_code_generator/__init__.py", line 507, in generate
    results = parser.parse()
  File "/path/.venv/lib/python3.13/site-packages/datamodel_code_generator/parser/base.py", line 1415, in parse
    self.__collapse_root_models(
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        models, unused_models, imports, scoped_model_resolver
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/path/.venv/lib/python3.13/site-packages/datamodel_code_generator/parser/base.py", line 1058, in __collapse_root_models
    model_field.constraints = ConstraintsBase.merge_constraints(
                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        root_type_field.constraints, model_field.constraints
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/path/.venv/lib/python3.13/site-packages/datamodel_code_generator/model/base.py", line 90, in merge_constraints
    if not issubclass(constraints_class, ConstraintsBase):  # pragma: no cover
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 123, in __subclasscheck__
TypeError: issubclass() arg 1 must be a class

To Reproduce

Schema https://csrc.nist.gov/schema/nvd/api/2.0/cve_api_json_2.0.schema

Used commandline:

$ datamodel-codegen --input ./schemas/cve-nvd/cve_api_json_2.0.schema.json --input-file-type jsonschema --output ~/Desktop/temp/ --output-model-type pydantic_v2.BaseModel --field-constraints --collapse-root-models

Expected behavior
Not an error

Version:

• OS: OSX
• Python version: 3.13
• datamodel-code-generator version: 0.25.6