[ty] Bind Self typevar to method context (#20366)
Fixes: https://github.com/astral-sh/ty/issues/1173
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## Summary
This PR will change the logic of binding Self type variables to bind
self to the immediate function that it's used on.
Since we are binding `self` to methods and not the class itself we need
to ensure that we bind self consistently.
The fix is to traverse scopes containing the self and find the first
function inside a class and use that function to bind the typevar for
self.
If no such scope is found we fallback to the normal behavior. Using Self
outside of a class scope is not legal anyway.
## Test Plan
Added a new mdtest.
Checked the diagnostics that are not emitted anymore in [primer
results](https://github.com/astral-sh/ruff/pull/20366#issuecomment-3289411424).
It looks good altough I don't completely understand what was wrong
before.
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Co-authored-by: Douglas Creager <dcreager@dcreager.net>
