openapi message_type is defined as an enum instead of a const

[lfse-slafleur]

The field message_type is currently defined as an enum in openapi.yml.

Example from line 807:

      properties:
        message_type:
          type: string
          enum: [ "DDBC.TimerStatus" ]

Rather a constant is expected such as:

        message_type:
          const: DDBC.TimerStatus
          type: string

A const is generated differently by generators such as datamodel-codegen. An enum will be generated to its own type and it is harder to use while a const will remain a simple string.