fix: validate number comparison operand types in num_lt, num_gt, num_lte, num_gte

Replace silent RETURN_NULL for non-numeric right operands with explicit runtime error via validateNum in four comparison primitives (num_lt, num_gt, num_lte, num_gte). Also remove redundant IS_NUM check in num_mod. Add four new test files verifying runtime error messages for each comparison operator when given a boolean operand.

test/number/greater_than_equal_operand_not_num.wren

@@ -0,0 +1 @@
+1 >= false // expect runtime error: Right operand must be a number.

test/number/greater_than_operand_not_num.wren

@@ -0,0 +1 @@
+1 > false // expect runtime error: Right operand must be a number.

test/number/less_than_equal_operand_not_num.wren

@@ -0,0 +1 @@
+1 <= false // expect runtime error: Right operand must be a number.

test/number/less_than_operand_not_num.wren

@@ -0,0 +1 @@
+1 < false // expect runtime error: Right operand must be a number.