fix(client): handle tokens without trades

[kartojal]

Returns None for a token without trades and documents the placeholder price response.

DEV-506

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Note

Medium Risk
Breaking change: callers must handle LastTradePrice | None on get_last_trade_price; behavior is limited to CLOB read parsing with no auth or trading impact.

Overview
get_last_trade_price now returns None when the CLOB responds with an empty side (the API’s placeholder for tokens that have never traded), instead of surfacing a bogus LastTradePrice.

Parsing goes through a new _LastTradePriceResponse in clob actions that accepts side: "" and maps it to None. Return types and docstrings are updated on public, secure, and async clients; get_last_trade_prices docs note that untraded tokens are omitted and results must be matched by token_id, not by input order.

The market_prices example prints N/A when there is no last trade. Unit/transport tests cover the no-trade response.

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