fix: filter non-arithmetic operators in ArithmeticMutator.CanMutate

ArithmeticMutator.CanMutate returned true for all BinaryExpr nodes
including conditional (==, !=) and logical (&&, ||) operators.
While getArithmeticMutations returned nil for these, the lack of
filtering caused unnecessary processing and could lead to bugs
when adding new mutators.

Add isArithmeticOp and isArithmeticAssignOp helper methods to
properly filter only arithmetic operators, consistent with how
ConditionalMutator and LogicalMutator handle their own operators.

Author: sivchari

internal/mutation/arithmetic.go

@@ -25,18 +25,36 @@ func (m *ArithmeticMutator) Name() string {
 
 // CanMutate returns true if the node can be mutated by this mutator.
 func (m *ArithmeticMutator) CanMutate(node ast.Node) bool {
-	switch node.(type) {
+	switch n := node.(type) {
 	case *ast.BinaryExpr:
-		return true
+		return m.isArithmeticOp(n.Op)
 	case *ast.AssignStmt:
-		return true
+		return m.isArithmeticAssignOp(n.Tok)
 	case *ast.IncDecStmt:
 		return true
 	}
 
 	return false
 }
 
+func (m *ArithmeticMutator) isArithmeticOp(op token.Token) bool {
+	switch op {
+	case token.ADD, token.SUB, token.MUL, token.QUO, token.REM:
+		return true
+	default:
+		return false
+	}
+}
+
+func (m *ArithmeticMutator) isArithmeticAssignOp(op token.Token) bool {
+	switch op {
+	case token.ADD_ASSIGN, token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN:
+		return true
+	default:
+		return false
+	}
+}
+
 // Mutate generates mutants for the given node.
 func (m *ArithmeticMutator) Mutate(node ast.Node, fset *token.FileSet) []Mutant {
 	var mutants []Mutant