Bring your own student levels to the exam generation embed

The student level dropdown in the exam generation embed used to offer only Examplary's built-in school levels. That's a poor fit if your learners are, say, beginner and advanced drivers. Pass availableStudentLevels in the session's presets and the dropdown shows only your levels instead:

{
  "presets": {
    "availableStudentLevels": [
      { "id": "beginner", "name": "Beginner driver", "instructions": "Use simple language and avoid jargon." }
    ]
  }
}

The instructions go into the AI prompt, so generated questions match the level your user picked. The chosen level is saved with the exam, so grading and feedback use it later too. Keep instructions to two or three sentences. See custom student levels for the full shape.