Deserializing error for string type

還沒有人認領這個 Issue。

**Describe the bug**
String types in service types cause microROS on the MCU side to fail in the executor due to a deserializing error. Running it through a debugger it fails in ucdr_deserialize_sequence_header when parsing the nested message type.

Service calls with non-string types work as expected. I've verified it's not a buffer overflow since the buffer size >> bytes received. There is also no packet corruption in UDP traffic, the dest packet and src packet match 1:1.

**To Reproduce**
Steps to reproduce the behaviour:
1. On MCU, init service with service type that has a string type
2. On host side, call service

**Expected behaviour**
The MCU accepts the service call and handles accordingly.

**Actual behaviour**
Executor fails due to deserialization error.

**System information (please complete the following information):**
- OS: Ubuntu 22.04
- ROS 2 Humble
- MCU: STM32F767ZI

使用 ROS 2 Humble，在 STM32F767ZI 上重現包含 string 欄位的服務呼叫，然後檢查剖析巢狀訊息型別時 ucdr_deserialize_sequence_header 處的 executor 失敗。當 MCU 能夠接受並處理該服務呼叫且不發生反序列化錯誤，同時不包含 string 的服務呼叫繼續正常運作時，該 issue 即完成。

由索引模型根據 Issue 內容生成。