Feature: Following STL bounds-check policy

### Describe what you are looking for

In integer-receiving interfaces like `basic_string_slice::find(string_view,size_type = 0)` and `find(value_type,size_type = 0)` we explicitly document UV if the requested slice is out of bounds, but STL returns an `npos`. We may want to change that to match STL more closely in the next releases.

### Can you contribute to the implementation?

- [x] I can contribute

### Is your feature request specific to a certain interface?

C++ bindings

### Contact Details

_No response_

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start with the C++ bindings for basic_string_slice::find(string_view, size_type = 0) and find(value_type, size_type = 0), then compare their documented out-of-bounds behavior with the STL. Done means the agreed bounds-check policy is applied consistently across the relevant integer-receiving interfaces and the behavior is documented.

Written by the indexing model from the issue text.