[BUG][Csharp][GenericHost] OpenApi 3.1: Model property of type map/additionalproperties gets generated as object

[gerwinjansen]

PR #17624 introduced an issue when generating code for a model with a property of type map/additionalproperties

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

Input

Response:
  type: object
  properties:
    success:
      type: boolean
    results:
      additionalProperties: #key-value map
        oneOf:
          - $ref: '#/components/schemas/FileResult'
          - $ref: '#/components/schemas/StringResult'
  required:
    - success
    - results

Expected output

public Response(bool success, Dictionary<string, ResponseResultsValue>? results = default)
{
    Success = success;
    Results = results;
    OnCreated();
}

Actual output

public Response(bool success, Object? results = default)
{
    Success = success;
    Results = results;
    OnCreated();
}

[wing328]

thanks for the PR, which looks good me.

after merging, i'll replace this test (sample) with a unit test instead similar to other 3.1 tests we've added so far.

[wing328]

done via #18828