fix(pop3): add NOOP command handler to POP3 server

[Au6ojlut]

First of all, thank you for the fantastic smtp4dev project!

This pull request adds support for the NOOP (No Operation) command to the POP3 server implementation, enhancing RFC 1939 protocol compliance.

Files Added:

NoopCommand.cs - New command handler implementing the NOOP operation

Files Modified:

Pop3Server.cs - Registered NOOP handler in the command handlers dictionary

Implementation Details (from RFC 1939)

      NOOP

         Arguments: none

         Restrictions:
             may only be given in the TRANSACTION state

         Discussion:
             The POP3 server does nothing, it merely replies with a
             positive response.

         Possible Responses:
             +OK

         Examples:
             C: NOOP
             S: +OK
             

[github-actions]

✅ All contributors have signed the CLA!

Thank you for signing the Contributor License Agreement. Your contribution can now be reviewed and merged! 🚀
_{Posted by the CLA Assistant Lite bot.}

[Au6ojlut]

I have read the CLA Document and I hereby sign the CLA

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rnwood]

@Au6ojlut Thanks for your contribution. I'll merge this once I look at the CI issue. Thanks.