Fix CallbackKey type for more accurate type checking

[ferruzzi]

Using a UUID stored as a raw string for the CallbackKey was short-sighted and almost bit us with #62343 also using a raw string for their id. If any future workloads also use a raw string, it would break all sorts of stuff. I'm following my own advice there and making CallbackKey a dataclass. This also lets us tighten up some other type checks where we assume "if it's not a TaskInstanceKey then it must be a CallbackKey".

Added some unit testing, tightened up some existing tests with our new and improved type-checking, and also manually tested by running a Dag with a synchronous deadline callback.

Tangentially related to #63491 but I don't think it'll cause any issues.

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[ferruzzi]

Tagging @dondaum for eyes since you've been doing a lot of work in this area.