Full options chain (calls + puts) for a US stock symbol and one expiry.

    Use this when the user asks "what's the options chain for X?", "show me
    AAPL puts expiring next Friday", or wants to inspect bid/ask/IV/volume on
    a specific strike. If no expiry is provided, returns the nearest expiry
    so Claude can quote it back and ask "want a different one?".

    Args:
        symbol: US stock symbol — AAPL, NVDA, TSLA, SPY, etc.
        expiry: Optional ISO date (YYYY-MM-DD). Must match one of the
            `available_expiries` Yahoo returns; otherwise returns an error
            with the list of valid dates.

    Returns:
        - underlying_price, underlying_change_pct
        - requested_expiry, available_expiries (list of YYYY-MM-DD)
        - call_count, put_count
        - calls: list of {strike, last_price, bid, ask, volume,
          open_interest, implied_volatility, in_the_money, expiration}
        - puts: same shape as calls