CommandHandler does not send required status responses in many cases

[bzbarsky-apple]

Problem

CommandHandler::ProcessCommandDataElement will send a status response in failure cases.

But if we don't reach that function, we will fail to send spec-required status responses. Specifically, if Command::ProcessCommandMessage errors out before calling ProcessCommandDataElement (which it can do if the command list is missing in the TLV or has the wrong tag), we will never reach ProcessCommandDataElement.

Proposed Solution

Add tests and fix. Perhaps hoist the status-response-sending to Command::ProcessCommandMessage?

[bzbarsky-apple]

So at this point the logic flow is:

1. InteractionModelEngine::OnMessageReceived will send a StatusResponse with Failure (not the right error code for our purposes here) if it calls something that fails.
2. It calls OnInvokeCommandRequest.
3. That calls CommandHandler::OnInvokeCommandRequest.
4. That calls CommandHandler::ProcessInvokeRequest.

We should either propagate the status bits down into all that or something.

Note also a similar TODO comment at the end of InteractionModelEngine::OnReadInitialRequest.

[yunhanw-google]

#21557