[BUG] [php] [php-nexgen] allOf with annotation-only schema and string generates missing MixedString type

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

A schema I'm consuming uses a weird way to add a description to a property.

When a schema property uses allOf to combine an annotation-only schema with a primitive string schema, the PHP generators produce a property type named MixedString.

No MixedString class is generated, so the generated client can fail at runtime when deserializing that field.

The expected generated PHP type for the property should be string, or the generator should otherwise handle the annotation-only schema without producing an unresolved model/type name.

openapi-generator version

• v7.24.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: anyOfTypeWithAnnotationOnlySchema
  description: Example of MixedString being generated with annotaiton only schema
  version: 1.0.0
paths:
  /:
    get:
      operationId: first
      responses:
        '200':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/foo'
          description: This route breaks
components:
  schemas:
    foo:
      type: object
      properties:
        bar:
          allOf:
            - $ref: '#/components/schemas/note'
            - type: string
    note:
      description: This is just a note. That's all it is

Generation Details

I used the following commands for generation:

openapi-generator-cli generate
    -i ./api.yml
    -g php-nextgen
    -c ./generate-nextgen.json
    -o ./out

openapi-generator-cli generate
    -i ./api.yml
    -g php
    -c ./generate-old.json
    -o ./out

Steps to reproduce

1. Generate using the steps above.
2. Check the generated Foo model.
3. See MixedString as type for the generated getter, setter, and openAPITypes for the field bar.

Related issues/PRs

I couldn't find anything.

Suggest a fix

I think empty schemas, as in this example should be filtered out for the type creation (the description should be combined?)
This should probably also be done for the anyOf path.

And maybe an allOf in combination with a primitive type should be disallowed. It's weird to have an allOf with a string and int.

Reproduce the issue with the provided OpenAPI YAML using both the php and php-nextgen generators, then inspect the generated Foo model and the generator paths handling allOf types. Done means the bar getter, setter, and openAPITypes use string without an unresolved MixedString type; consider the noted anyOf case as well.