Bug: Optionally typed parameters do not render in OpenAPI docs properly

[JacobCoffee]

Description

When using an arbitrary enum as a type (or Literal assumedly), it generates a dropdown in the OpenAPI docs.

environment: Environment = Parameter(
        title="Environment",
        description="The environment to filter by.",
        default=None,
        required=False,
    ),

but hitting this endpoint gives a validation error:

 File ".venv/lib/python3.10/site-packages/litestar/_signature/model.py", line 190, in parse_values_from_connection_kwargs
    return convert(kwargs, cls, strict=False, dec_hook=deserializer, str_keys=True).to_dict()
msgspec.ValidationError: Expected `str`, got `null` - at `$.environment`

This makes sense, but the fix of adding | None moves the dropdown field to just a plain textfield.

URL to code causing the issue

No response

MCVE

from enum import StrEnum
from litestar import Litestar, get
from litestar.params import Parameter

class Environment(StrEnum):
    DEVELOPMENT = "dev"
    TEST = "test"
    QA = "qa"

@get(path="/")
async def test(
    environment: Environment
    | None = Parameter(
        title="Environment",
        description="The environment to filter by.",
        default=None,
        required=False,
    ),
) -> None:
    return None

app = Litestar([test])

Steps to reproduce

1. Write mcve.py
2. Browse to OpenAPI schema
3. Observe dropdown fields
4. Execute on the test route
5. See error
6. Apply fix `Environment | None`
7. Reload OpenAPI schema
8. See no dropdown, just text field
9. Execute on the test route
10. See it work, but be deeply saddened that there is no longer a dropdown box and contemplate deleting your entire application.

Screenshots

: Environment

: Environment | None

Logs

No response

Litestar Version

2.x

Platform

• Linux
• Mac
• Windows
• Other (Please specify in the description above)

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[JacobCoffee]

Without looking, this is probably expected behavior.. but it sucks

[peterschutt]

    environment: Environment
    | None = Parameter(

"rough fmt" amiright?