Delete a contact by phone

Permanently deletes the contact identified by the E.164 phone number under the authenticated account. The account ID is taken from the bearer token. Requires a concrete current ETag in If-Match; the wildcard If-Match value is rejected with 400, and a stale ETag returns 412. Opt-outs are preserved by Contacts Service so re-importing the phone does not resubscribe it. Returns 204 with no response body, including when no contact has the phone, so repeating a delete is safe.