Only apply validation on content update to variant cultures where the editor has permission for the culture

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Validation errors are currently reported for cultures that exist on a content item but that the editor doesn't have permissions for.

With this PR I've ensured that we provide the appropriate cultures to validate based on the current backoffice user's permissions.

To Test:

You can see this with the following setup:

• Create a document type that varies by culture.
• Create a single text string property, that also varies by culture, and mark it as mandatory.
• Create a second language.
• Create a user group that has access only to the default language.
• Create a user that is only in this user group.
• Make the following request to the management API via Swagger when authenticated as this editor (replacing the document ID, property alias and language as appropriate for your setup):

PUT /umbraco/management/api/v1.1/document/77c5256e-1298-4f48-911c-ba0787ceb5d8/validate
{
   "template":null,
   "values":[
      {
         "editorAlias":"Umbraco.TextBox",
         "culture":"en-US",
         "segment":null,
         "alias":"message",
         "value":"Test message"
      }
   ],
   "variants":[
      {
         "culture":"en-US",
         "segment":null,
         "state":"Published",
         "name":"Test Page",
         "publishDate":"2025-03-24T07:04:00.38+00:00",
         "createDate":"2025-03-24T07:04:00.193+00:00",
         "updateDate":"2025-03-24T07:04:00.38+00:00",
         "scheduledPublishDate":null,
         "scheduledUnpublishDate":null
      }
   ]
}

• With the current v15/dev you'll get this response:

400 Bad Request
{
  "type": "Error",
  "title": "Validation failed",
  "status": 400,
  "detail": "One or more properties did not pass validation",
  "operationStatus": "PropertyValidationError",
  "errors": {
    "$.values[?(@.alias == 'message' && @.culture == 'it' && @.segment == null)].Value": [
      "#validation_invalidNull"
    ]
  }
}

• With the code from this PR applied, you should get:

200 Success