WAMP Flatbuffers serialization test coverage; WAMP message classes refactoring

## Description - Add [WAMP Flatbuffers serialization test coverage](https://github.com/crossbario/autobahn-python/issues/1771) - Internal refactoring (no API change) of WAMP message classes - Add docs for WAMP Message Class Design --- ## Related Issue(s) Closes or relates to #1771 --- ## Checklist - [x] I have referenced relevant issue numbers above - [x] I have performed a self-review of my code and it follows the style guidelines of this project - [x] I have added new or used existing tests that prove my fix is effective or that my feature works - [x] I have added necessary documentation (if appropriate) and updated the changelog - [x] I have added an AI assistance disclosure file (required!) in this PR