"""Usage metric helpers for WebUI display payloads. Prompt-cache hit percentage is cached prompt reads over the full prompt total(input + cache reads + cache writes). Keep this calculation in the backend sobrowser display code cannot drift across context indicator and per-turn labels."""  def _to_int(value) -> int:    try:        return int(value or 0)    except (TypeError, ValueError):        return 0  def prompt_cache_hit_percent(cache_read_tokens, prompt_tokens):    """Return cached reads as a percent of full prompt-token total.     ``prompt_tokens`` must include ordinary input, cache reads, and cache writes    (matching Agent's ``session_prompt_tokens`` value).    """    cache_read = _to_int(cache_read_tokens)    prompt = _to_int(prompt_tokens)    if cache_read <= 0 or prompt <= 0:        return None    return min(100, round((cache_read / prompt) * 100))