feat: Enhance security and robustness of Rust ipv6-tester

[MitchellJThomas]

This commit introduces several improvements to the Rust implementation of the ipv6-tester tool to address potential security flaws and improve its overall robustness.

Key changes include:

1. Improved Address Parsing: The IPv6 address parsing in IPv6Tester::run now handles invalid addresses gracefully by returning an error, rather than panicking. This prevents unexpected server/client termination due to malformed default or user-supplied IP addresses.

2. Newline-Based Message Framing: The handle_client server logic has been refactored to use BufReader::read_line. This ensures that messages are processed based on newline delimiters, making the server correctly interpret messages that might arrive in chunks or are sent rapidly.

3. Message Size Limit: A maximum message length (MAX_LINE_LENGTH set to 4096 bytes) is now enforced in handle_client. If a client sends a message exceeding this limit, the server logs an error, attempts to notify the client, and then closes the connection. This mitigates a potential denial-of-service vector.

4. Logging Review: Logging statements were reviewed. While existing logging is suitable for a developer tool, a comment was added to main.rs to remind developers to consider log sensitivity in different operational contexts.

5. Documentation:

   • In-code comments were added/updated to explain the rationale behind these security and robustness enhancements.
   • The README.md has been updated with a new "Security Considerations" section. This section informs you about the implemented protections (address validation, message framing, size limits), the tool's intended scope, its lack of encryption/authentication, and general advice for safe usage.

These changes collectively make the Rust ipv6-tester more resilient against common issues and more transparent about its security characteristics.