Implement better and more consistent incoming message validation in all message handlers

[turbocrime]

Message listeners perform inconsistent validation of incoming messages, and test coverage is poor.

different listeners may validate input with

• type guards
• helper functions
• string literal comparison
• an enumerated switch
• object inspection code directly in the handler

Type guards tend to be colocated with message type definitions, so are not obviously critical to message validation in listeners. These should be documented and called out with docstrings.

Object inspection code inside handlers should be converted to type guards with docstrings, and relocated with the message types.

Some existing helper methods could be applied more broadly.

Some validation functions have test coverage, but presently, all listeners and type guards have zero test coverage. These paths are security critical and should have full test coverage, with special attention to:

• malformed input
• input that satisfies multiple types
• listeners expecting input from specific sources