`const` keyword does not exist in OpenAPI 3.0.3 and needs to be replaced by `enum`

[BITespresso]

Using Tapir to generate the OpenAPI 3.0.3 specification for coproducts with a discriminator field, the value of the discriminator value for the coproducts has the type const in the OpenAPI 3.0.3 specification.

Unfortunately const was introduced in OpenAPI 3.1 and does not exist in OpenAPI 3.0.3. What needs to be done for OpenAPI 3.0.3 is to use a single enum value instead (see https://www.speakeasy.com/openapi/schemas/enums#constants-for-single-value-enums).

I prepared a REPL do demonstrate this:

scala-cli -S 3.6.4 \
  --dep com.softwaremill.sttp.apispec::jsonschema-circe:0.11.9 \
  --dep com.softwaremill.sttp.apispec::openapi-circe:0.11.9 \
  --dep com.softwaremill.sttp.tapir::tapir-apispec-docs:1.11.28

when running

import io.circe.Codec as CirceCodec
import io.circe.derivation.Configuration as CirceConfiguration
import io.circe.syntax.*
import sttp.apispec.circe.*
import sttp.tapir.docs.apispec.schema.TapirSchemaToJsonSchema
import sttp.tapir.generic.Configuration
import sttp.tapir.*

sealed trait Entity
case class Person(firstname: String, lastname: String) extends Entity
case class Organization(name: String) extends Entity

val discriminatorFieldName = "kind"

given Configuration = Configuration.default.withDiscriminator(discriminatorFieldName)
given CirceConfiguration = CirceConfiguration.default.withDiscriminator(discriminatorFieldName)

given CirceCodec[Entity] = CirceCodec.AsObject.derivedConfigured
given Schema[Entity] = Schema.derived

import sttp.apispec.openapi
import sttp.apispec.openapi.circe_openapi_3_0_3._

TapirSchemaToJsonSchema(summon[Schema[Entity]], markOptionsAsNullable = true).asJson

the resulting JSON contains a const value in the discriminator field:

  "$defs" : {
    "Organization" : {
      "title" : "Organization",
      "type" : "object",
      "required" : [
        "name",
        "kind"
      ],
      "properties" : {
        "name" : {
          "type" : "string"
        },
        "kind" : {
          "type" : "string",
          "const" : "Organization"
        }
      }
    }

However the expected result for OpenAPI 3.0.3 would be:

  "$defs" : {
    "Organization" : {
      "title" : "Organization",
      "type" : "object",
      "required" : [
        "name",
        "kind"
      ],
      "properties" : {
        "name" : {
          "type" : "string"
        },
        "kind" : {
          "type" : "string",
          "enum": ["Organization"]
        }
      }
    }

I'll submit a PR which fixes this.