fix: Livechat Custom Fields displaying only visitor scope - #40325

[R21-R23] Prototype pollution: The filter condition on line 24 checks `value` as a truthy boolean but doesn't check if `key` is an own property of `userCustomFields`. Object.entries() enumerates enumerable properties from the prototype chain in addition to own properties, so an attacker who can control the `userCustomFields` object can inject prototype properties like `__proto__`, `constructor`, or `prototype` as keys. When the filter condition `checkIsVisibleAndCorrectScope(key, data?.customFields, scope) && value` evaluates, these prototype keys will retrieve values from the Object prototype (like function references), which are truthy and can cause the prototype-related entries to be returned as valid custom fields. This can overwrite inherited properties across the process. Fix by adding a `.hasOwnProperty()` check before filtering, e.g., use Object.entries(userCustomFields).filter(([key, value]) => Object.prototype.hasOwnProperty.call(userCustomFields, key) && checkIsVisibleAndCorrectScope(key, data?.customFields, scope) && value).