[Testing] Add network and P2P layer tests

## Problem

Current test coverage:
- **Total tests**: 433
- **Network/P2P tests**: 0

No tests for:
- Message propagation
- Peer discovery
- Connection handling
- Network partitions
- Byzantine scenarios

## Risk

- Network failures undetected until production
- Message loss handling untested
- Reconnection logic unverified

## Required Tests

1. Basic peer connectivity
2. Message broadcast/receive
3. Connection drop and reconnect
4. Network partition simulation
5. Malformed message handling
6. Rate limiting behavior

## Affected Crates

- `crates/consensus` (network layer)
- `crates/data-chain` (primary/worker communication)
- `crates/node` (peer management)