test: add test case for memiterator.go (#22725)

Co-authored-by: heren-ke <fghjtuy@163.com>

store/cachekv/internal/memiterator_test.go

@@ -0,0 +1,73 @@
+package internal
+
+import (
+	"testing"
+)
+
+func TestMemIterator_Ascending(t *testing.T) {
+	db := NewBTree()
+	// db.set()
+	db.Set([]byte("a"), []byte("value_a"))
+	db.Set([]byte("b"), []byte("value_b"))
+	db.Set([]byte("c"), []byte("value_c"))
+
+	iterator := newMemIterator([]byte("a"), []byte("c"), db, true)
+
+	var result []string
+	for iterator.Valid() {
+		result = append(result, string(iterator.Key()))
+		iterator.Next()
+	}
+
+	expected := []string{"a", "b", "c"}
+	for i, key := range result {
+		if key != expected[i] {
+			t.Errorf("Expected %s, got %s", expected[i], key)
+		}
+	}
+
+	if iterator.Valid() {
+		t.Errorf("Iterator should be invalid after last item")
+	}
+}
+
+func TestMemIterator_Descending(t *testing.T) {
+	db := NewBTree()
+
+	db.Set([]byte("a"), []byte("value_a"))
+	db.Set([]byte("b"), []byte("value_b"))
+	db.Set([]byte("c"), []byte("value_c"))
+	db.Set([]byte("d"), []byte("value_d"))
+
+	iterator := newMemIterator([]byte("a"), []byte("d"), db, false)
+
+	var result []string
+	for iterator.Valid() {
+		result = append(result, string(iterator.Key()))
+		iterator.Next()
+	}
+
+	expected := []string{"c", "b", "a"}
+	for i, key := range result {
+		if key != expected[i] {
+			t.Errorf("Expected %s, got %s", expected[i], key)
+		}
+	}
+
+	if iterator.Valid() {
+		t.Errorf("Iterator should be invalid after last item")
+	}
+}
+
+func TestMemIterator_EmptyRange(t *testing.T) {
+	db := NewBTree()
+	db.Set([]byte("a"), []byte("value_a"))
+	db.Set([]byte("b"), []byte("value_b"))
+	db.Set([]byte("c"), []byte("value_c"))
+
+	iterator := newMemIterator([]byte("d"), []byte("e"), db, true)
+
+	if iterator.Valid() {
+		t.Errorf("Iterator should be invalid for empty range")
+	}
+}