wrap setitem/setattr assign failures in PathAssignError

[Sahana2524]

_assign_op only turned failures into PathAssignError for the registry ('P') branch, so an assignment through a T-expression destination leaked the raw error instead:

• Assign(T[i], v) past the end of a list, or into an immutable tuple, surfaced a bare IndexError/TypeError wrapped by the generic trace
• Assign(T.attr, v) onto a read-only attribute did the same with AttributeError
• the string/Path form already raised PathAssignError, and Delete._del_one wraps all three op branches, so this was the odd one out

Wrapped the setitem and setattr branches the same way, which also matches what the Assign docstring already promises. Test covers the list, tuple, and attribute cases.