Get Schedule By Index

Return one previously retained schedule without advancing generation.

Args: schedule_id: Unique identifier of the containing generation session. index: Zero-based index into schedules generated so far.

Returns: The stored serialized schedule, requested index, and current total count.

Raises: HTTPException: Status 404 when either the session or requested index does not exist; the detail identifies the currently available index interval.

Behavior: Results are read directly from the append-only serialized schedule list. No scheduler initialization or generation lock is required, and retrieving a result refreshes the session's idle-expiry timestamp.

ScheduleResponse(schedule_id='...', schedule=[...], index=0, total_generated=1)