## Add a Contact

Add a new contact for the merchant. Either contact_email or contact_mobile must be provided.
The service will check for existing contacts with the same ID, email, or mobile number
and raise a conflict error if any duplicates are found.

## Endpoint signature

```http
POST https://api.reservepay.com/merchants/add-contact HTTP/1.1
Content-Type: application/json
Accept: application/json
Authorization: Bearer <token>

{
  external_id: string?,
  contact_email: string?,
  contact_mobile: string?,
  contact_name: string?,
  favorite: boolean?,
}
```

Returns: string

## Request arguments

### `external_id` (string)

_Optional_. The external ID for the contact. Optional but must be unique if provided.

### `contact_email` (string)

_Optional_. The email address for the contact. Either `contact_email` or `contact_mobile` must be provided.

### `contact_mobile` (string)

_Optional_. The mobile number for the contact. Either `contact_email` or `contact_mobile` must be provided. Dashes and spaces are accepted and will be stripped automatically.

### `contact_name` (string)

_Optional_. The name of the contact.

### `favorite` (boolean)

_Optional_. Whether the contact is a favorite. Optional, defaults to false.

## Errors specific to this endpoint

### `CONFLICT`

A contact with the same ID, email, or mobile number already exists.
Each contact must have unique values for these fields.

### `VALIDATION_ERROR`

Validation failed. Either contact_email or contact_mobile must be provided.
If contact_email is provided, it must be a valid email format.

## Errors common to all endpoints

### `UNHANDLED_ERROR`

This error occurs when the server encounters an unexpected internal error that
it cannot handle gracefully. This typically happens due to bugs, infrastructure
issues, or edge cases that weren't anticipated during development.

### `INVALID_ARGUMENTS`

This error occurs when the request contains invalid or missing parameters.
Common cases include missing required fields, or values that don't match the
expected format or type.

### `BAD_VERSION`

This error occurs when making requests to an API version that does not exist.
This commonly happens when using an outdated SDK or when the API version
specified in the request URL is incorrect.