Add typed error handling, retries, and webhook receiver

- Typed exceptions: LeadMailException base with LeadMailRequestException
  (structured statusCode/errorCode/logId/validationErrors) and
  LeadMailConnectionException; sendEmail/getDomains now throw these instead
  of raw Guzzle exceptions, and a malformed body is no longer a silent null.
- Automatic retry with exponential backoff on idempotent calls
  (getDomains, verifyEmail); sends are never retried to avoid duplicates.
- Webhook receiver: auto-registered route + LeadMailWebhookController that
  verifies the HMAC signature, logs failures, and dispatches a
  LeadMailWebhookReceived event. WebhookSignature/WebhookEvent/LeadMailWebhook
  helpers for manual handling.
- Webhook self-registration client methods (registerWebhook/getWebhook/
  deleteWebhook) and a promptless `leadmail:install` command that registers
  the URL and writes LEADMAIL_WEBHOOK_SECRET to .env.
- Null-safe client binding when LEADMAIL_TOKEN is unset.
- Test suite (Pest + Testbench) covering all of the above.

tests/TestCase.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace LeadM\LeadMail\Tests;
+
+use LeadM\LeadMail\LeadMailServiceProvider;
+use Orchestra\Testbench\TestCase as Orchestra;
+
+abstract class TestCase extends Orchestra
+{
+    /**
+     * @return array<int, class-string>
+     */
+    protected function getPackageProviders($app): array
+    {
+        return [LeadMailServiceProvider::class];
+    }
+}