Send SMS with Java

Add SMS sending to this Java project with TextKit.

- Docs for this stack: https://jointextkit.com/docs/send-with-java.md
- API: POST https://api.jointextkit.com/v1/messages with `Authorization: Bearer
  <key>` and a JSON body `{"to": "+1...", "body": "..."}`.
- There is no Java SDK: POST JSON to https://api.jointextkit.com/v1/messages
  with java.net.http.HttpClient.
- Read the key from the `TEXTKIT_API_KEY` environment variable. Never hardcode
  it or send it to the browser.
- Phone numbers are E.164 and US or Canada only (`+1` followed by 10 digits).
- A test key (`ssms_sk_test_`) simulates: `+15005550006` always ends `delivered`
  and no phone gets a text.
- Errors are JSON: `{"error": {"code", "message", "param"}}` with a 4xx status.
  On `429`, wait `Retry-After` seconds and retry.
- Send only from server code that runs after a real event (a signup, an order).
  Never expose an endpoint that texts any number a visitor types.

export TEXTKIT_API_KEY=ssms_sk_test_...

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

public class SendSms {
    public static void main(String[] args) throws Exception {
        String json = """
            {"to": "+15005550006", "body": "Hello from TextKit"}
            """;

        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://api.jointextkit.com/v1/messages"))
            .header("Authorization", "Bearer " + System.getenv("TEXTKIT_API_KEY"))
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(json))
            .build();

        HttpResponse<String> response = HttpClient.newHttpClient()
            .send(request, HttpResponse.BodyHandlers.ofString());

        if (response.statusCode() >= 400) {
            System.err.println(response.statusCode() + " " + response.body());
            System.exit(1);
        }
        System.out.println(response.body());
    }
}

java SendSms.java