fix: avoid panic on undersized packets in Vortex::try_from

[immanuwell]

Description

Guard Vortex::try_from before split_to(HEADER_SIZE), so inputs shorter than 6 bytes return InvalidPacket instead of blowing up.

Also adds a regression test for a 3 byte packet. tiny fix, but a real footgun.

Related Issue

Closes #102

Motivation and Context

This is triggerable in practice. Read::read can return fewer bytes than requested, so partial or truncated buffers are normal enough.

Repro on main:

use bytes::Bytes;
use vortex_protocol::Vortex;

let _ = Vortex::try_from(Bytes::from_static(&[1, 2, 3]));

Before this patch it panics in BytesMut::split_to. After this patch it returns invalid vortex packet. pretty straight forward.

Screenshots (if appropriate)

n/a

[gaius-qi]

LGTM