reduce is_subset_eq_var size

https://github.com/facebook/pyrefly/blob/fb7004fb993c92927a5264de237daf64ef601c55/pyrefly/lib/solver/solver.rs#L3993

has ~400LoC, (Var, Type) and (Type, Var) are mirror

how about keep Var/Var, and make other like

```rs
fn solve_var_against(
&mut self,
var: Var,
other: &Type,
side: BoundSide,
) -> Result<(), SubsetError>
```

Lower for `other <: var`, Upper for `var <: other`

Start in pyrefly/lib/solver/solver.rs around line 3993 and inspect the existing is_subset_eq_var implementation, especially the mirrored (Var, Type) and (Type, Var) cases. Compare how BoundSide is used for lower and upper bounds, then confirm that the refactoring preserves the existing solver behavior and reduces the duplicated implementation.

Written by the indexing model from the issue text.