feat: add error packet handling.

[deskchen]

This pull request enhances error packet handling in both the proxy and proxy-buffer components, ensuring error packets are correctly detected, processed, forwarded, and tested. The changes introduce a dedicated method for error packet processing, improve routing information propagation, and add comprehensive tests for error packet serialization, deserialization, and validation.

Error Packet Handling Improvements:

• Added a ProcessErrorPacket method to PacketBuffer in both cmd/proxy/buffer.go and cmd/proxy-buffer/buffer.go, enabling direct processing and forwarding of error packets without fragmentation. This method constructs a BufferedPacket with full routing information. [1] [2]
• Updated the handlePacket logic in both cmd/proxy/main.go and cmd/proxy-buffer/main.go to detect error packets by their type ID, process them using the new method, and forward them directly to the intended peer, bypassing the element chain. [1] [2]

Error Packet Routing and Serialization:

• Modified SendErrorPacket in cmd/proxy/util/send_packet.go and cmd/proxy-buffer/util/send_packet.go to include source and destination IP/port fields in error packets, ensuring accurate routing information is preserved and transmitted.
• Updated all usages of SendErrorPacket to pass the new routing parameters. [1] [2]

Testing Enhancements:

• Added extensive tests for error packet serialization/deserialization, MTU validation, and the new error packet handling logic in both cmd/proxy/main_test.go and cmd/proxy/buffer_test.go. These tests verify correct field propagation, error detection, and boundary conditions. [1] [2]

Other Changes:

• Set the default log level to "debug" in cmd/proxy/main.go to aid in development and troubleshooting.
• Added missing imports for the packet package where required. [1] [2]

These changes collectively improve the reliability and observability of error handling in the proxy system, ensuring error packets are robustly managed and tested.