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.

composer.json

@@ -17,6 +17,8 @@
     "require": {
         "php": "^8.2",
         "guzzlehttp/guzzle": "^7.0",
+        "illuminate/console": "^11.0|^12.0|^13.0",
+        "illuminate/http": "^11.0|^12.0|^13.0",
         "illuminate/mail": "^11.0|^12.0|^13.0",
         "illuminate/support": "^11.0|^12.0|^13.0"
     },
@@ -45,5 +47,10 @@
         }
     },
     "minimum-stability": "dev",
-    "prefer-stable": true
+    "prefer-stable": true,
+    "config": {
+        "allow-plugins": {
+            "pestphp/pest-plugin": true
+        }
+    }
 }