`PathToDirectory` can throw for valid S3 URIs without `?`

> `PathToDirectory`: for `s3://...` paths without a '?' query string, `find_last_of('?')` returns `npos` and `path.substr(t)` will throw `std::out_of_range`. Handle the `npos` case explicitly (e.g., treat it as no suffix) to avoid crashing on valid S3 URIs.
> ```suggestion
> std::string prefix;
> std::string suffix;
> if (t == std::string::npos) {
> prefix = path;
> suffix.clear();
> } else {
> prefix = path.substr(0, t);
> suffix = path.substr(t);
> }
> ```

_Originally posted by @Copilot in [#878](https://github.com/apache/incubator-graphar/pull/878/changes#r2858598451)_

Locate the C++ PathToDirectory entry point and inspect how it splits an S3 URI around the '?' character. Reproduce the no-query-string case, handle it without throwing, and verify that URI handling with and without a suffix still behaves as expected using the relevant existing tests.

Written by the indexing model from the issue text.