Reject invalid Qwen logits and discard poisoned sessions

src/engine/metal/qwen_mtplx/fused.rs

@@ -51,6 +51,7 @@ pub(super) fn operator(op: &Operation) -> Option<&'static str> {
         },
         Operation::Binary(op) => match op {
             B::Equal => "Equal",
+            B::NotEqual => "NotEqual",
             B::GreaterEqual => "GreaterEqual",
             B::Greater => "Greater",
             B::Less => "Less",