Deserializing error for string type

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**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 上复现包含 string 字段的服务调用，然后检查解析嵌套消息类型时 ucdr_deserialize_sequence_header 处的 executor 故障。当 MCU 能够接受并处理该服务调用且不发生反序列化错误，同时不包含 string 的服务调用继续正常工作时，该 issue 即完成。

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