test: add checksum tests for package y (#2246)

**Description**

Added `y/checksum_test.go` with unit tests for:
CRC32C calculation (including empty input)
XXHash64 calculation
VerifyChecksum success cases
VerifyChecksum mismatch case
CalculateChecksum panic on unsupported algorithm

Author: miladev95

y/checksum_test.go

@@ -0,0 +1,56 @@
+package y
+
+import (
+	"hash/crc32"
+	"testing"
+
+	"github.com/cespare/xxhash/v2"
+	"github.com/dgraph-io/badger/v4/pb"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCalculateChecksum_CRC32C(t *testing.T) {
+	data := []byte("hello world")
+	expected := uint64(crc32.Checksum(data, CastagnoliCrcTable))
+	got := CalculateChecksum(data, pb.Checksum_CRC32C)
+	require.Equal(t, expected, got)
+
+	// empty input
+	expectedEmpty := uint64(crc32.Checksum([]byte{}, CastagnoliCrcTable))
+	gotEmpty := CalculateChecksum([]byte{}, pb.Checksum_CRC32C)
+	require.Equal(t, expectedEmpty, gotEmpty)
+}
+
+func TestCalculateChecksum_XXHash64(t *testing.T) {
+	data := []byte("hello world")
+	expected := xxhash.Sum64(data)
+	got := CalculateChecksum(data, pb.Checksum_XXHash64)
+	require.Equal(t, expected, got)
+}
+
+func TestVerifyChecksum_Success(t *testing.T) {
+	data := []byte("hello world")
+	c1 := &pb.Checksum{Algo: pb.Checksum_CRC32C, Sum: CalculateChecksum(data, pb.Checksum_CRC32C)}
+	require.NoError(t, VerifyChecksum(data, c1))
+
+	c2 := &pb.Checksum{Algo: pb.Checksum_XXHash64, Sum: CalculateChecksum(data, pb.Checksum_XXHash64)}
+	require.NoError(t, VerifyChecksum(data, c2))
+}
+
+func TestVerifyChecksum_Mismatch(t *testing.T) {
+	data := []byte("x")
+	c := &pb.Checksum{Algo: pb.Checksum_CRC32C, Sum: 0}
+	err := VerifyChecksum(data, c)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "checksum mismatch")
+}
+
+func TestCalculateChecksum_UnsupportedAlgoPanics(t *testing.T) {
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatalf("expected panic for unsupported algorithm")
+		}
+	}()
+
+	_ = CalculateChecksum([]byte("x"), pb.Checksum_Algorithm(999))
+}