This commit adds the remaining error handling tests for the SMTP mail sending functionality.

- Adds a test to ensure `MissingRecipientsError` is raised when no recipients are provided for an admin email.
- Adds a test to ensure `InvalidSenderError` is raised for SMTP authentication errors.
- Adds a test to ensure a generic `Error` is raised for SMTP connection errors and missing `SMTP_HOST` configuration.
- Corrects the exception type raised in the `send()` method to use the base `Error` class for transient SMTP errors.

This change prefixes all environment variables related to the external SMTP mail service with `APPENGINE_` to improve namespacing and prevent potential conflicts in the deployment environment.

The following variables have been renamed:
- USE_SMTP_MAIL_SERVICE -> APPENGINE_USE_SMTP_MAIL_SERVICE
- SMTP_HOST -> APPENGINE_SMTP_HOST
- SMTP_PORT -> APPENGINE_SMTP_PORT
- SMTP_USER -> APPENGINE_SMTP_USER
- SMTP_PASSWORD -> APPENGINE_SMTP_PASSWORD
- SMTP_USE_TLS -> APPENGINE_SMTP_USE_TLS
- ADMIN_EMAIL_RECIPIENTS -> APPENGINE_ADMIN_EMAIL_RECIPIENTS

The changes have been applied to both the SDK implementation in `mail.py` and the corresponding unit tests in `mail_unittest.py