Convert string_view to string before forwarding to underlying C API

[SpriteOvO]

Closes #716.

When I use the WebView to load HTML, I've noticed that a vertical scroll bar occasionally appears on the right side, but this behavior isn't consistently reproducible. After a build, unexpected garbage characters always appear at the end of the HTML, which is the cause of the scrollbar. This is obviously a buffer overflow.

The problem is that C++'s std::string_view does not guarantee that it is '\0' terminated.

cppref:

Unlike std::basic_string::data() and string literals, std::basic_string_view::data() returns a pointer to a buffer that is not necessarily null-terminated, for example a substring view (e.g. from remove_suffix). Therefore, it is typically a mistake to pass data() to a routine that takes just a const CharT* and expects a null-terminated string.

In my use case, the HTML comes from a char array generated by xxd, which is not '\0' terminated. After it is passed to webui, subsequent calls to strlen and further string operations lead to a buffer overflow.

If strings themselves come from std::string or char const [], they will not be affected by this vulnerability.

With the assistance of AI, I successfully wrote a PoC. If the parameters come from user-controlled input, under specific usage scenarios, the resulting buffer overflow can eventually reach the system call, enabling multiple RCE exploits.

This PR is a quick mitigation. If we want to preserve the zero-copy advantage of std::string_view, we may need to modify the C API to accept an additional length parameter.