fix: Narrow typevar bounds in isinstance check

[tannguyencse19]

Summary

When narrowing encounters a solver Var that represents a quantified type parameter, preserve it as Type::Quantified instead of forcing it to its bound.
This allows isinstance narrowing to compute an anonymous intersection like T & int, which remains assignable to the original T.

Fixes #1274

Test Plan

cargo test -p pyrefly test_isinstance_narrow_bounded_type_param --lib