Descriptor Protocol — Validated Attribute

Implement a PositiveNumber descriptor that:

• Allows setting only positive numbers (> 0). Raises ValueError for zero or negatives.
• Returns the stored value on get.
• Stores values per-instance (not on the descriptor itself).

class Circle:
    radius = PositiveNumber()

c = Circle()
c.radius = 5    # OK
c.radius        # → 5
c.radius = -1   # raises ValueError

Implement solve(value) that creates a Circle, sets its radius = value and returns the radius. If value <= 0, return the string "error".