feat(string): add digest length validation for DelEX IFDEQ/IFDNE

[kirito632]

• Add 16-character digest validation for IFDEQ/IFDNE options in DelEX command
• Return clear error if digest length is not exactly 16 hexadecimal characters
• Add test case for invalid digest length rejection as requested by reviewer

This commit only modifies DelEX command behavior.

[jihuayu]

Redis Stack handles uppercase scenarios; please add this test case.

	t.Run("DelEX IFDEQ and IFDNE accept uppercase digest", func(t *testing.T) {
		key := "test-string-key-uppercase-digest"
		value := "Hello world"
		var digest string

		require.NoError(t, rdb.Del(ctx, key).Err())
		require.NoError(t, rdb.Set(ctx, key, value, 0).Err())
		digest = strings.ToUpper(rdb.Do(ctx, "DIGEST", key).Val().(string))
		require.Equal(t, int64(1), rdb.Do(ctx, "DelEX", key, "ifdeq", digest).Val())
		require.Equal(t, int64(0), rdb.Exists(ctx, key).Val())

		require.NoError(t, rdb.Set(ctx, key, value, 0).Err())
		digest = strings.ToUpper(rdb.Do(ctx, "DIGEST", key).Val().(string))
		require.Equal(t, int64(0), rdb.Do(ctx, "DelEX", key, "ifdne", digest).Val())
		require.Equal(t, value, rdb.Get(ctx, key).Val())
	})