PYTHON-5676 Phase 3: Unify run_operation() with network.command()

Extract _network_command_core() from network.command() to handle the
common send/receive/APM/decrypt pipeline. Both the standard command
path and the cursor (find/getMore) path now use the same core function.

- New _network_command_core() in network.py: takes a pre-built message
  and handles APM, send_message, receive_message, unpack, process,
  check, decrypt — returns (docs, reply, duration)
- command() is now a thin wrapper: encode → _network_command_core → return docs[0]
- run_operation() is now a thin wrapper: build message → _network_command_core
  → wrap in Response/PinnedResponse
- _CURSOR_DOC_FIELDS moved from server.py to network.py (only used in core)
- server.py drops _decode_all_selective, _check_command_response,
  _convert_exception, NotPrimaryError, OperationFailure imports

Author: aclark4life