I don't think CodeRunner will be able to work in the way you hope. CodeRunner uses a sandbox server called Jobe that provides a service to the Moodle server via a RESTful interface. That URL you quote, http://localhost/jobe/index.php/restapi/languages, is part of that API but it's not accessible to the questions that are being tested.
You ask about where the students' code is saved. Each run is saved in a temporary workspace on the Jobe server, which for security reasons is inaccessible to any other processes on the server except the program itself (i.e. the program consisting of the question author's marking code plus the student code).
A CodeRunner question must consist of the student's code wrapped in author-supplied test code via the question's template. You could possibly have testing code that sets the $_GET and $_POST superglobals to mimic the behaviour of a web service calling their code, and you could then capture and grade their response, but you can't run any web services as part of this process.
You ask about where the students' code is saved. Each run is saved in a temporary workspace on the Jobe server, which for security reasons is inaccessible to any other processes on the server except the program itself (i.e. the program consisting of the question author's marking code plus the student code).
A CodeRunner question must consist of the student's code wrapped in author-supplied test code via the question's template. You could possibly have testing code that sets the $_GET and $_POST superglobals to mimic the behaviour of a web service calling their code, and you could then capture and grade their response, but you can't run any web services as part of this process.