[parbhatkapila4]

When a Nexus start or cancel handler fails, the failure is encoded inside the except BaseException block. If the task gets cancelled during that encoding, for example, when the worker shuts down, the CancelledError escapes the block. The outer except Exception doesn't catch it, so _complete_task() is never called and core never gets a completion for the task. Worker shutdown then waits for it forever, which is what the comment on _complete_task() warns about.

Now a cancel during that encode sends an ack_cancel completion, the same thing a cancel during the success path's encode already does.

Added a test with a payload codec that is slow to encode, so worker shutdown cancels the task while it's encoding the failure. It covers the start handler and the cancel handler. On main, shutdown doesn't finish within 10 seconds in either case. With the fix, both pass. The rest of tests/nexus gives the same results with and without this change.

[tconley1428]

Raw sleep should be removed before completion.