Bug: Enum OAS issues

[wallseat]

Description

I have two problems with how Enum declared in OAS.

1. When I create 3 different models with same enum, i find, that in always paste in schema As Is. There is not schema declared in components->schemas section of OAS.
   It cause a problem with frontend, when they create bindings with code-gen tools. They need to handle this strings directly.
   I think it is better to register Enum as objects in schemas and refer to them.

2. When you try to make an Enum field optional, instead of getting oneOf{null, schema{enum{...}} you get enum{..., null}, which is incorrect, i think. Because i'm not realy declared null as value of this enum.
   This problem caused by the same logic as above.

I have code to fix it (we use it as monkey-patch in our project), so if we confirm that this is a problem, i'll provide PR.

URL to code causing the issue

No response

MCVE

from enum import StrEnum

from litestar import Litestar, get
from litestar.openapi import OpenAPIConfig
from litestar.openapi.plugins import JsonRenderPlugin
from pydantic import BaseModel, Field


class TestEnum(StrEnum):
    """ "Description!"""

    ONE = "one"
    TWO = "two"


class TestSchemaA(BaseModel):
    enum_field: TestEnum | None = Field(default=None)
    other_filed: int | None = Field(default=None)


class TestSchemaB(BaseModel):
    enum_field: TestEnum
    other_field: int


@get("/a")
async def test_routerA() -> TestSchemaA:
    return TestSchemaA(enum_field=TestEnum.ONE)


@get("/b")
async def test_routerB() -> TestSchemaB:
    return TestSchemaB(enum_field=TestEnum.TWO)


app = Litestar(
    route_handlers=[test_routerA, test_routerB],
    openapi_config=OpenAPIConfig(
        path="/docs",
        title="Test Api",
        version="0.1.0",
        render_plugins=[JsonRenderPlugin()],
    ),
)

Result schema

{
  "info": {
    "title": "Test Api",
    "version": "0.1.0"
  },
  "openapi": "3.1.0",
  "servers": [
    {
      "url": "/"
    }
  ],
  "paths": {
    "/a": {
      "get": {
        "summary": "TestRoutera",
        "operationId": "ATestRoutera",
        "responses": {
          "200": {
            "description": "Request fulfilled, document follows",
            "headers": {

            },
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/TestSchemaA"
                }
              }
            }
          }
        },
        "deprecated": false
      }
    },
    "/b": {
      "get": {
        "summary": "TestRouterb",
        "operationId": "BTestRouterb",
        "responses": {
          "200": {
            "description": "Request fulfilled, document follows",
            "headers": {

            },
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/TestSchemaB"
                }
              }
            }
          }
        },
        "deprecated": false
      }
    }
  },
  "components": {
    "schemas": {
      "TestSchemaA": {
        "properties": {
          "enum_field": {
            "type": [
              "null",
              "string"
            ],
            "enum": [
              "one",
              "two",
              null]
          },
          "other_filed": {
            "oneOf": [
              {
                "type": "null"
              },
              {
                "type": "integer"
              }
            ]
          }
        },
        "type": "object",
        "required": [],
        "title": "TestSchemaA"
      },
      "TestSchemaB": {
        "properties": {
          "enum_field": {
            "type": "string",
            "enum": [
              "one",
              "two"
            ]
          },
          "other_field": {
            "type": "integer"
          }
        },
        "type": "object",
        "required": [
          "enum_field",
          "other_field"
        ],
        "title": "TestSchemaB"
      }
    }
  }
}

Litestar Version

2.8.2

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[provinzkraut]

@wallseat Your suggestions sound reasonable, feel free to submit a PR with the proposed changes :)