date-time format incorrectly allows integers in generated Pydantic model

[schelv]

Describe the bug
When the OpenAPI specification uses type: string with format: date-time, the generated Pydantic model allows integer values for the date-time field, even though according to the OpenAPI specification (and RFC 3339), only valid ISO 8601 date-time strings should be accepted. This means values like TestObject(-1) or TestObject(1234) are incorrectly accepted as valid date-time.
See OpenAPI and RFC 3339.

To Reproduce

Example :

openapi: 3.1.1
info:
  title: Test API
  version: 1.0.0
paths: {}
components:
  schemas:
    TestObject:
      description: My test object
      type: string
      format: date-time

using this command

$ datamodel-codegen --input openapi.yaml --input-file-type openapi --output openapi.py --output-model-type pydantic_v2.BaseModel

this is generated:

from __future__ import annotations

from pydantic import AwareDatetime, Field, RootModel


class TestObject(RootModel[AwareDatetime]):
    root: AwareDatetime = Field(..., description='My test object')

With this model, it is possible to do:

TestObject(-1)  # This should raise a validation error but does not.

Expected behavior
When the OpenAPI specification uses format: date-time, the generated Pydantic model should only accept valid ISO 8601 date-time strings.

Only valid ISO 8601 strings should be accepted for the date-time variant, and negative integers should not be accepted at all.

Version:

• OS: Linux
• Python version: 3.12
• datamodel-code-generator version: 0.33.3