[ty] Simplify TypeVar assignability/subtyping logic (#24138)
## Summary
Refactor the TypeVar type relation checking logic to simplify the
control flow and eliminate redundant conditions. The change consolidates
two separate match arms that handled inferable TypeVars into a single
arm with conditional logic based on whether the relation is an
assignability check.
## Test Plan
Existing tests
Co-authored-by: Claude <noreply@anthropic.com>
