PYTHON-5781 Increase coverage for network_layer.py

Add test/test_network_layer.py with 56 unit tests covering:
- PyMongoProtocol: process_header (including ProtocolError paths),
  process_compression_header, get_buffer (all state branches),
  buffer_updated (state machine), close/connection_lost lifecycle
- NetworkingInterfaceBase: abstract method NotImplementedError raises
- NetworkingInterface: socket delegation methods
- AsyncNetworkingInterface: transport/protocol delegation
- sendall: trivial delegation
- _async_socket_receive: success and connection-closed paths

Author: aclark4life