fix: resolve writelock contention in DoraChannel receive_p2p (#202)

[AdityaShome]

Summary

Fixes a deadlock in DoraChannel::receive_p2p where a write lock on the receivers mawas held across an async rx.recv().await, blocking all concurrent callers for up to 30 seconds.
Fixes: #202 and continues #212

Changes:

• Changed receivers map value type from mpsc::Receiver to Arc<Mutex<mpsc::Receiver>>
• Updated receive_p2p to acquire a brief read lock, clone the Arc, drop the map lock,
  then await on the per-agent mutex only
• Updated try_receive_p2p with the same pattern using .lock.await()
• Updated register_agent to wrap the new receiver in Arc::new(Mutex::new(rx))
• Added test_p2p_receive_deadlock to reproduce and verify the fix

Testing

The added test test_p2p_receive_deadlock spawns a blocking receive and concurrently calls register_agent, asserting it completes in under 400 ms. It failed before the fix and passes after.