fix: correct Mailgun API URL and enhance error handling in MailgunAccount class

- Fixed the incorrect API URL in MailgunAccount class.
- Improved error handling for email sending failures by providing detailed error messages based on response content.
- Updated test file to export the tap.start() function for better module integration.

package.json

@@ -9,17 +9,17 @@
   "author": "Lossless GmbH",
   "license": "MIT",
   "scripts": {
-    "test": "(tstest test/)",
+    "test": "(tstest test/ --verbose)",
     "build": "(tsbuild --web --allowimplicitany)",
     "format": "(gitzone format)",
     "buildDocs": "tsdoc"
   },
   "devDependencies": {
     "@git.zone/tsbuild": "^2.1.65",
-    "@git.zone/tstest": "^2.5.0",
+    "@git.zone/tstest": "^2.6.1",
     "@push.rocks/qenv": "^6.1.3",
     "@push.rocks/tapbundle": "^6.0.3",
-    "@types/node": "^24.7.2"
+    "@types/node": "^22.0.0"
   },
   "dependencies": {
     "@push.rocks/smartfile": "^11.2.7",