[BUG][CSharp] type array uniqueItems uses invalid type

Description

In the generichost CSharp generator, if there's a response that is of the type array and contains the property "uniqueItems": true, the generated file will use Set<T> (which is non-existent) instead of a valid type such as HashSet<T>.

openapi-generator version

7.16.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.1",
  "info": {
    "title": "CSharp Code Generation Error Test",
    "description": "This API is made with the purpose of replicating the csharp code generation issue with openapi-generators.",
    "version": "1.0.0"
  },
  "servers": [
    {
      "url": "/"
    }
  ],
  "paths": {
    "/api/test": {
      "get": {
        "summary": "Test endpoint",
        "description": "This is a test endpoint to replicate the csharp code generation issue with openapi-generators",
        "responses": {
          "200": {
            "description": "Successful Request",
            "content": {
              "application/json": {
                "schema": {
                  "uniqueItems": true,
                  "type": "array",
                  "items": {
                    "$ref": "#/components/schemas/Test"
                  }
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Test": {
        "type": "object",
        "properties": {
          "id": {
            "type": "integer",
            "format": "int64"
          },
          "type": {
            "type": "string"
          },
          "label": {
            "type": "string"
          }
        }
      }
	}
  }
}

Generation Details

npx --yes @openapitools/openapi-generator-cli generate -i ./test.json -g csharp -o test --additional-properties=library=generichost,apiName=TestApi,packageName=TestApi,useSourceGeneration=true,sourceFolder=.,targetFramework=net9.0

Steps to reproduce

• Generate a CSharp project with the provided schema above.

Related issues/PRs

None that I could find.

Suggest a fix

Provide specific C# compatible types when uniqueItems is specified, such as HashSet

Run the provided npx openapi-generator command with the supplied OpenAPI declaration and inspect the generated CSharp generichost output for the response array. Trace how uniqueItems is mapped to its collection type, then verify that the generated project uses a valid C# type and compiles successfully.