Increase test coverage in critical areas -- S110

The task here is to analyze current test coverage numbers, and wherever critical code paths are not covered adequately, add new unit or scenario tests to cover them. For this task, it is not a goal to increase test coverage artificially by adding tests for simple areas already observably correct via code inspection (e.g. negative tests with invalid parameters). But we need to ensure we have adequate coverage of:

sync and async code paths through all channel layers
code paths supporting code generated by Connected Services
untested pivots of supported message encoders, bindings, and transports
error handling a developer could encounter in mainline scenarios
Tasks (may be repeated area-by-area):
[ ] Analyze code coverage and identify important areas lacking test coverage
[ ] Add unit or scenario test to cover them