Try reflected binary-op dunder first for proper subclasses (#3876)

[durvesh1992]

Summary

Fixes #3876.

Per Python's data model, when the right operand's type is a proper subclass of the left operand's type, the reflected dunder is tried first. Pyrefly always tried the forward dunder first, so for example:

from enum import IntFlag
class Color(IntFlag):
    RED = 1
    GREEN = 2

def f(x: int, c: Color):
    reveal_type(x & c)  # was: int   — should be: Color

int & Color resolved through int.__and__ (which widens the result back to int) instead of Color.__rand__ (which returns the flag type Self). At runtime int_val & color calls Color.__rand__ and yields a Color.

Fix

In binop_types' binop_call (pyrefly/lib/alt/operators.rs), when the RHS type is a proper subclass of the LHS type, the reflected dunder is now tried first.

This is intentionally narrow:

• It only reorders when rhs is a strict subclass of lhs (has_superclass + not equal).
• A subclass that does not override the reflected dunder inherits it unchanged, and try_binop_calls still falls back to the forward dunder if the reflected call doesn't apply — so non-overriding subclasses are unaffected.

Test

Added test_reflected_dunder_subclass_priority in pyrefly/lib/test/operators.rs asserting x & c, x | c, x ^ c are Color for x: int, c: Color. It fails before this change (inferred int) and passes after.