Real-time pre-market and after-hours prices for a US stock symbol.

    Use this when the user asks about a stock outside the regular 9:30am-4pm
    ET session — earnings reactions, overnight news, "what is X doing in
    after-hours?", "how did Y open in pre-market?". Returns the most recent
    valid print from each session window (pre-market, regular, post-market)
    along with computed % changes vs. the previous close and the regular
    close, respectively.

    During the regular session, post_market will be null (no data yet).
    On weekends/holidays, returns whatever's most recent in each window.

    Args:
        symbol: US stock symbol — AAPL, NVDA, TSLA, SPY, ^GSPC, etc.

    Returns:
        - pre_market: {price, as_of_utc, change_vs_previous_close_pct} or null
        - regular: {price, as_of_utc, change_pct} (consolidated tape close)
        - post_market: {price, as_of_utc, change_vs_regular_close_pct} or null
        - previous_close, currency, exchange, market_state for context