[BUG][csharp-netcore] Nullable but required properties not respected

Bug Report Checklist

• 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

If I have a property that is required but nullable, the generated code throws during deserialization when the value is null.

openapi-generator version

6.5.0

OpenAPI declaration file content or url

spec:

"properties": {
	"deletedAt": {
		"type": "string",
		"format": "date-time",
		"nullable": true
	},
}
"required": [
	"deletedAt"
]

Generation Details

generated code:

// to ensure "deletedAt" is required (not null)
if (deletedAt == null)
{
    throw new ArgumentNullException("deletedAt is a required property for XXX and cannot be null");
}

Steps to reproduce

Related issues/PRs

Suggest a fix

Start from the csharp-netcore generator using the OpenAPI declaration in this issue and trace the generated deserialization check shown in the report. Reproduce with a required, nullable deletedAt date-time property; done means null is accepted for that property instead of throwing, while required-property behavior remains correct.