Honor RemotePort for SNMP sessions

[SoundGoof]

Summary

Fixes #4 by making the RemotePort session parameter control the destination UDP port used for SNMP requests.

Root Cause

RemotePort was parsed and stored on the Python Session object, but it was never passed to the native session constructors. As a result, requests continued to use the default SNMP port 161.

Net-SNMP no longer uses its native remote_port session field. The supported method is to include the destination port in the peer name.

Changes

• Append a non-default RemotePort to the native UDP peer name.
• Apply the peer name to SNMP v1, v2c, and v3 UDP sessions.
• Leave tunneled TLS, DTLS, and SSH transports unchanged.
• Preserve support for the existing DestHost="host:port" syntax.
• Add packet-level regression tests using an ephemeral UDP listener.

Testing

• Built the native extension on Fedora 43 with Python 3.14 and Net-SNMP 5.9.4.
• Verified that RemotePort sends the SNMP request to the selected port.
• Verified that the existing DestHost="host:port" syntax continues to work.
• Confirmed both regression tests pass.