Fix UDP receive for binary SIP bodies

[yeoleobun]

Summary

• Parse incoming UDP datagrams as SIP message bytes instead of requiring the entire datagram to be valid UTF-8.
• Keep debug logging readable with lossy UTF-8 rendering.
• Add a UDP regression test for a SIP MESSAGE carrying a binary application/isup body.

Root cause

UDP receive decoded &buf[..len] with std::str::from_utf8 before parsing. SIP headers are text, but the body is opaque bytes, so valid SIP messages with binary bodies were dropped before reaching the parser.

Validation

• cargo test transport::tests::test_udp

Fixes #131