Add unit tests for SOCKS5 UDP header generation and parsing

[zh-jq]

Motivation

• Verify correctness of SOCKS5 UDP header generation and parsing for IPv4, IPv6, IPv4-mapped IPv6 and domain names.
• Ensure reusable header buffer behavior is correct and that malformed packets are rejected with proper errors.

Description

• Add a #[cfg(test)] mod tests with unit tests exercising UdpOutput::generate_header, UdpOutput::generate_header2, and UdpInput::parse_header for round-trip encoding/decoding of IPv4, IPv6, and domain addresses.
• Add a test that confirms an IPv4-mapped IPv6 address is encoded using the IPv4 header format and that UDP_HEADER_LEN_IPV4 is used.
• Add a test validating the SocksUdpHeader::encode behavior where the internal buffer resizes for a domain address and the returned view shrinks for subsequent IPv4 encoding.
• Add tests that UdpInput::parse_header rejects several malformed packets and returns the expected SocksUdpPacketError variants.

Testing

• Ran the crate unit tests with cargo test and verified the new tests execute successfully against the vey-socks UDP header logic.
• All added tests passed (no failures reported).

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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