fix: preserve binary message bodies when serializing to the wire

[WaelJami]

Summary

SIP message bodies are opaque octets (RFC 3261 §7.4), but rsipstack serialized them as UTF-8 text, corrupting any body that isn't valid UTF-8. This makes to_bytes() the canonical wire serializer (preserving the body verbatim) and routes the UDP/TCP/TLS transports through it.

Problem

A message with a binary body, e.g. an application/ISUP part, or an eCall MSD (binary ASN.1 PER), is sent with the body mangled. Every non-UTF-8 byte is replaced by U+FFFD (EF BF BD), which both changes the payload and breaks Content-Length.

Root cause

All message→bytes paths went through Display/to_string(), which renders the body with String::from_utf8_lossy:

// src/sip/message.rs (Display for Request/Response)
write!(f, "...{}\r\n{}", self.headers, String::from_utf8_lossy(&self.body))

and the transports serialized via to_string() (transport/udp.rs, transport/stream.rs).

Notably, the receive path already preserves binary bodies (the parser slices the body by Content-Length), so send and receive were asymmetric.

Fix

• Add to_bytes() to Request, Response, and SipMessage: start line + headers as ASCII, body appended verbatim.
• From<…> for Vec<u8> now delegates to to_bytes().
• UDP send, the TCP/TLS SipCodec encoder, and send_to_stream use to_bytes().

Scope

WebSocket transport is intentionally unchanged: SIP-over-WS (RFC 7118 §5) uses UTF-8 text frames, so binary bodies aren't representable there regardless.

Testing

• New regression tests: a Request/Response round-trip and the stream codec, each asserting a non-UTF-8 body survives serialization byte-for-byte.
• Full suite: cargo test --lib → 238 passed, clippy clean.

Compatibility

Backward compatible. Display/to_string() behaviour is unchanged; only the byte serialization used by transports is corrected.