[BUG][JAVA][PYTHON] Warnings displayed for discriminator property if "$ref" is used.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When using OAPI3 "discriminator" support, openapi-generator generates warnings when discriminator properties are specified with "$ref".

This is not ideal for large APIs that wish to use a shared enum that contains the discriminator mappings for many objects.

It is possible to silence the warning by re-defining the discriminator's type property in each polymorphic API object. In the example spec posted, this would look like:

BarBazType:
    type: string
    description: A discriminator reference.
    enum:
      - BAR
      - BAZ
      
Baz:
  required:
    - type
  properties:
     type:
        type: string
        $ref: "#/components/schemas/BarBazType"

However, this produces an incorrect client, because the type property is generated as StrictStr with no enum constraints.

Actual output
... [main] WARN o.o.codegen.DefaultCodegen - 'Foo' defines discriminator 'type', but the referenced schema 'Bar' is incorrect. invalid type for type, set it to string [main] WARN o.o.codegen.DefaultCodegen - 'Foo' defines discriminator 'type', but the referenced schema 'Baz' is incorrect. invalid type for type, set it to string ...

Expected output
No warnings.

openapi-generator version

Bug detected on 7.2.0, confirmed against master (hash 6f01a7ad823).

OpenAPI declaration file content or url

Minimal spec to reproduce:
https://gist.github.com/Hnefi/86cb2d748748b468b449ed4984e71bef

Generation Details

Command-line parameters to reproduce:

openapi-generator generate -g python -i oapi3.yaml -o /tmp/python_client

Steps to reproduce

1. Download the minimal API spec in the gist.
2. Generate the client with the above command.
3. Inspect warnings generated for the issue.

Related issues/PRs

The most relevant issue is:

• https://github.com/OpenAPITools/openapi-generator/issues/17893

Although the warning generated is the same, the root cause in my case seems to be the fact that the discriminator property in my minimal example uses "$ref".

Suggest a fix

When scanning the spec file to find the discriminator's property, inherit the type from child "$refs".

Start with the minimal OpenAPI spec in the linked gist and run the documented Python generation command to reproduce the DefaultCodegen warnings. Trace discriminator-property handling when the property uses $ref, including the related issue 17893. Done means generation produces no warnings and preserves the referenced enum constraints in the client.