SMTP module: Fix response code and Cyrillic character handling issues

[butschster]

Two issues were identified with the SMTP server:

1. When terminating the DATA command with the end-of-data marker (a single period), the server incorrectly sent two "250" response codes in sequence, causing errors with some clients during concurrent email sending. #274

2. When sending emails with Cyrillic content, especially with larger volumes of text, clients would receive an error: Expected response code "354" but got code "250". This was due to improper handling of multi-byte characters and SMTP state management.

Changes Made

1. In Service.php:

• Fixed the response handling for data termination to only send one "250" response
• Properly reset message state when a new DATA command is received
• Reset the waitBody flag when a message is successfully processed
• Improved handling of the SMTP protocol state machine

2. In Message.php:

• Updated string handling functions to use multi-byte aware methods (mb_substr, mb_strlen)
• Improved the end-of-stream detection to handle multi-byte characters properly
• Enhanced the getBody method to safely handle message termination

Testing

• Tested concurrent email sending to verify only one response code is sent
• Verified that emails with varying amounts of Cyrillic text can be sent without error
• Confirmed protocol compliance by checking response codes at each step of the SMTP transaction