Categorize KeyExpect uses and use distinct enum variants #2208
Description
Better Engineering task
I ran into a panic while working on a new feature caused by the fact that Pyrefly's architecture assumes we will never have two KeyExpects with the same location.
This is obviously not a very scalable invariant - it's a global properly of binding logic, impossible to verify by just skimming code. I know that my feature ran into a case where this is violated, but it's actually entirely possible that edge cases already exist which are just extremely rare that cause crashes.
We should just use an enum with good tags for different kinds of expectations to make this problem vastly less likely.
Assigning to myself - I can do this, but it has to happen after some of my current work is merged
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