fix(dcutr): add uvarint length-delimited framing to /libp2p/dcutr protocol messages

[SilverFire]

Summary

The current /libp2p/dcutr implementation in dart_libp2p (lib/p2p/protocol/holepunch/service.dart and holepuncher.dart) sends and expects raw Protobuf payload bytes without length-delimited message framing.

However, the libp2p DCUtR protocol specification and standard go-libp2p implementation (github.com/libp2p/go-libp2p/p2p/protocol/holepunch) frame stream messages using pbio (unsigned varint length prefix).

Problem Impact

When a dart_libp2p node attempts DCUtR hole punching with a go-libp2p peer over a circuit relay:

1. go-libp2p sends a CONNECT message prefixed by a uvarint length byte (for example, 0x1e).
2. dart_libp2p performs HolePunch.fromBuffer() on the unstripped stream, failing to parse the Protobuf tag.
3. The stream is reset, breaking the NAT hole-punching upgrade.

Fix

• Wrap outbound HolePunch Protobuf messages using encodeDelimitedMessage() to prepend the uvarint length.
• Use BufferedP2PStreamReader in _incomingHolePunch and _initiateHolePunchImpl to read the uvarint length prefix followed by the exact payload bytes.
• Cover single-byte and multi-byte uvarint prefixes and fragmented stream reads with unit tests.