fix: client should not specify taskId if it doesn't exist

[njbrake]

Description

I'm submitting this PR as a proposal based off of #218 (comment).

If the taskId doesn't exist, I was thinking that an exception should be thrown to prevent the client from specifying the id. The reason I think this change is required because if the client specifies a taskId but one doesnt exist, it won't actually get push notifications based off that first request, because the logic to set_info() gets skipped because there is no task when _setup_message_execution is called.

Found while working on mozilla-ai/any-agent#414

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Fixes #<issue_number_goes_here> 🦕