Describe the bug
When we use schema $def tag and define the subschemas separately if the first field in the object has a const value, the value is not detected but the object's name appears in the literal of the generated code.

To Reproduce
Create a schema such as given below.
Then generate the pydantic_v2 model using a command as follows

Example schema:

{
  "title": "fruits",
  "type": "object",
  "properties": {
    "families": {
      "type": "object",
      "discriminator": { "propertyName": "colour" },
      "oneOf": [{ "$ref": "#/$defs/apple" }, 
      { "$ref": "#/$defs/orange" }]
    }
  },
  "$defs": {
    "apple": {
      "type": "object",
      "properties": {
        "flavor": {
          "const": "sweet"
        }
      }
    },
    "orange": {
      "type": "object",
      "properties": {
        "flavor": {
          "const": "sour"
        }
      }
    }
  }
}

Used command line:

$ datamodel-codegen  --input schema.json --input-file-type jsonschema --output datamodel.py --output-model-type pydantic_v2.BaseModel

Expected behaviour
Expected

class Fruits(BaseModel):
    flavour: Literal['sweet'] = Field(
        ...
    )

What I got

class Fruits(BaseModel):
    flavour: Literal['apple'] = Field(
        ...
    )