Parser does not Dereference Discriminator Mapping Targets

When providing a discriminator for subtypes you can provide a reference to the target type for each discriminator value (see https://swagger.io/docs/specification/data-models/inheritance-and-polymorphism/)

I would expect this to be correctly resolved to the target schema object. Instead it is treated as a string and left as a reference. This means the Discriminator.getMapping() method returns data that is useless for deserializing a response.

The following OpenAPI json definition for the discriminator:

"discriminator": {
  "propertyName": "myDiscriminator",
  "mapping": {
    "type_a": "#/components/schemas/TypeA",
    "type_b": "#/components/schemas/TypeB"
  }
},

Produces a Discriminator instance (after python name mangling) like the following:

{
  "mapping": {
    "type_a": "#/components/schemas/TypeA",
    "type_b": "#/components/schemas/TypeB"
  },
  "propertyName": "my_discriminator"
}

I would expect the values of the mapping HashMap to actually be the target classes, so that this Discriminator object can be used to dispatch to the correct subclass.

Start by tracing Discriminator.getMapping() and the parser's reference-resolution path using the OpenAPI JSON example in the issue. Add a regression test for discriminator mappings and verify that mapping values resolve to target schema objects and support response deserialization dispatch.