Implements typed async wrappers for the four endpoints tuimail needs:
- list_messages: GET /mail/v4/messages (paged inbox listing)
- get_message: GET /mail/v4/messages/{id} (full message with encrypted body)
- delete_messages: PUT /mail/v4/messages/delete (soft-delete to Trash)
- get_public_keys: GET /core/v4/keys (recipient keys for outbound mail)
All responses decoded through Envelope<T> with Code 1000 check.
main.rs smoke-tests the inbox listing after authentication.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
