Fix UserWarning in console with MongoDsn type

[jag-k]

Warning in console:

/.../pydantic-settings-export/lib/python3.12/site-packages/pydantic/type_adapter.py:390: UserWarning: Pydantic serializer warnings:
  Expected `multi-host-url` but got `str` - serialized value may not be as expected
  return self.serializer.to_json(

Pydantic Model:

from pydantic import Field, MongoDsn
from pydantic_settings import BaseSettings


class AppSettings(BaseSettings):
    """Pydantic class for Ulla Bot Spawner settings."""

	mongo_uri: MongoDsn = Field("mongodb://localhost:27017/", description="MongoDB connection uri")