[BUG] [C#] Generator adds unnecessary IsSet validation for nullable fields not in required list

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

In OpenAPI specifications, the optional nature of a field can be expressed in two ways:

Not listed in the required array: This indicates that the field is optional.
Explicitly marked as nullable: true: This indicates that the field can accept null values.
However, when a field is not listed in the required array, it is already considered optional by definition. Explicitly adding nullable: true in such cases may be redundant.

Generation Details

Despite this, the OpenAPI Generator generates code that includes unnecessary IsSet validation logic for nullable fields. For example:

[JsonPropertyName("role")]
public string? Role { get; set; }

The string? type indicates that the field is nullable.
However, the generator still adds IsSet validation logic:

if (role.IsSet && role.Value == null)
    throw new ArgumentNullException(nameof(role), "Property is not nullable for class UserLoginSuccessDto.");

This behavior is inconsistent with the semantics of OpenAPI and leads to overly strict validation.

Steps to reproduce

In openapi def file:

role:
  type: string
  nullable: true

(!: role is not in required fields list)

use csharp generator and check auto-generated class file.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/4816

Suggest a fix

######Expected Behavior
If a field is not listed in the required array, it should be treated as optional without additional [IsSet] validation.
If the field is explicitly marked as nullable: true, the generator should respect this and allow null values without throwing exceptions.

######Actual Behavior
The generator adds unnecessary [IsSet] validation logic for nullable fields, even when they are not required.
This results in overly strict validation that contradicts the OpenAPI specification.

The key is: generator determine a field not in required list, so generator mark this field as nullable in cs def

string? Role

but with a confusing validation, unless mark this field with nullable: true.

Start with the C# generator output for the provided OpenAPI field and compare its handling of required and nullable properties with related issue #4816. Done means an optional nullable field such as role remains string? without generating the unnecessary IsSet validation or ArgumentNullException logic.