Fix undefined reference errors for StringToVector with GCC and LTO.

This CL addresses link failures when building MuJoCo with GCC and Link Time Optimization (LTO) enabled:
- Declared the `StringToVector<std::string>(const std::string&)` specialization in `user_util.h` to prevent the compiler from incorrectly trying to instantiate the general template.
- Added an explicit instantiation for `StringToVector<unsigned char>(char*)` in `user_util.cc` to provide the definition needed by its `const std::string&` counterpart.

PiperOrigin-RevId: 902557141
Change-Id: I27b585cf1deb276d4150a5cbe6cf7e59c323a20a

src/user/user_util.cc

@@ -1346,6 +1346,7 @@ template<> MJAPI std::vector<std::string> StringToVector(const std::string& s) {
 template MJAPI std::vector<int>    StringToVector(char* cs);
 template MJAPI std::vector<float>  StringToVector(char* cs);
 template MJAPI std::vector<double> StringToVector(char* cs);
+template MJAPI std::vector<unsigned char> StringToVector(char* cs);
 
 
 template <typename T> std::vector<T> StringToVector(const std::string& s) {

src/user/user_util.h

@@ -264,6 +264,7 @@ template<typename T> MJAPI std::string VectorToString(const std::vector<T>& v);
 // convert string to vector
 template<typename T> MJAPI std::vector<T> StringToVector(char *cs);
 template<typename T> MJAPI std::vector<T> StringToVector(const std::string& s);
+template<> MJAPI std::vector<std::string> StringToVector(const std::string& s);
 
 }  // namespace mujoco::user