QuickBooks Customer Upsert

Create a new QuickBooks customer or update if they already exist. Searches for existing customer by email or display name, then either updates the found customer or creates a new one. Use this when you need to ensure a customer exists with specific information, such as adding a customer from a phone call, updating contact details, or ensuring no duplicate customers are created. Only provide the fields you want to set/update - null fields are ignored. If both email and display_name are provided, email takes precedence for searching. Returns the created or updated customer object.