toJson abstract method

Serializes this resource to a JSON-encodable map.

Returns:

• A map containing all relevant resource metadata
• Should include at minimum a 'type' field identifying the resource kind
• Must contain only JSON-encodable values

Implementation Requirements:

• Must be overridden by subclasses
• Should include all identifying metadata
• Should maintain backward compatibility

Example Output:

{
  "type": "font",
  "family": "Roboto",
  "files": ["Roboto-Regular.ttf", "Roboto-Bold.ttf"]
}