[lldb-dap] Test Gardening, improving DebugCommunication. (llvm#141689)

Improving the readability and correctness of DebugCommunication by
adding type annotations to many parts of the library and trying to
improve the implementation of a few key areas of the code to better
handle correctness.

Specifically, this refactored the
`DebugCommunication._handle_recv_packet` function to ensure consistency
with the reader thread when handling state changes and improved the
`DebugCommunication._recv_packet` helper to make it easier to follow by
adding some additional helpers.

Author: ashgti