[BUG] [CSHARP] Models from schema with a property attribute "nullable": true are generating properties not nullable

Bug Report Checklist

• [X ] 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 properties are configured to be both required and nullable, the generated C# model does not allow a null value for that property.

openapi-generator version

Used
"@openapitools/openapi-generator-cli": "^2.18.4"
with

openapi-generator-cli version-manager set 7.12.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.4",
  "info": {
    "title": "Required Issue Example",
    "version": "v1"
  },
  "paths": {
    "/api/Example": {
      "post": {
        "tags": [
          "Example"
        ],
        "requestBody": {
          "content": {
            "application/json-patch+json": {
              "schema": {
                "allOf": [
                  {
                    "$ref": "#/components/schemas/Example"
                  }
                ]
              }
            },
            "application/json": {
              "schema": {
                "allOf": [
                  {
                    "$ref": "#/components/schemas/Example"
                  }
                ]
              }
            },
            "text/json": {
              "schema": {
                "allOf": [
                  {
                    "$ref": "#/components/schemas/Example"
                  }
                ]
              }
            },
            "application/*+json": {
              "schema": {
                "allOf": [
                  {
                    "$ref": "#/components/schemas/Example"
                  }
                ]
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "OK"
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Example": {
        "required": [
          "requiredNullable",
          "requiredNotNullable"
        ],
        "type": "object",
        "properties": {
          "requiredNullable": {
            "type": "string",
            "nullable": true
          },
          "requiredNotNullable": {
            "type": "string"
          }
        },
        "additionalProperties": false
      }
    },
    "securitySchemes": {
      "JWT": {
        "type": "http",
        "description": "Please enter in a valid token",
        "scheme": "Bearer",
        "bearerFormat": "Json Web Token,"
      }
    }
  }
}

Generation Details

The problem is that the property "requiredNullable" does not allow the null value, even though it is configured in the json as:

"properties": {
  "requiredNullable": {
    "type": "string",
    "nullable": true
    },
...

Constructor generated:

public Example(string requiredNullable = default(string), string requiredNotNullable = default(string))
{
    // to ensure "requiredNullable" is required (not null)
    if (requiredNullable == null)
    {
        throw new ArgumentNullException("requiredNullable is a required property for Example and cannot be null");
    }
    this.RequiredNullable = requiredNullable;
    // to ensure "requiredNotNullable" is required (not null)
    if (requiredNotNullable == null)
    {
        throw new ArgumentNullException("requiredNotNullable is a required property for Example and cannot be null");
    }
    this.RequiredNotNullable = requiredNotNullable;
}

This block (from above) will demand it to be not nullable:

if (requiredNullable == null)
    {
        throw new ArgumentNullException("requiredNullable is a required property for Example and cannot be null");
    }

Steps to reproduce

This is the command used to generate the Example.cs class:

openapi-generator-cli generate -i open-api-issue.json -g csharp -o ./example-client -p disallowAdditionalPropertiesIfNotPresent=false -p targetFramework=net8.0 -p packageName=ExampleClient -p netCoreProjectFile=true -p nullableReferenceTypes=true -p useCollection=true -p library=httpclient -p useSingleRequestParameter=true

Related issues/PRs

No

Suggest a fix

if the property is flagged with "nullable": true in the ref json , the generated C# model should not enforce the parameter to not be nullable.

Start by running the supplied openapi-generator-cli command with the embedded specification and inspect the generated example-client/Example.cs. Compare requiredNullable with requiredNotNullable when nullableReferenceTypes=true; done means the nullable property accepts null while the non-nullable required property remains enforced, with regression coverage for the reproduced case.