feat: USD-M futures WebSocket URL category support (#1684)

* feat: Add USD-M futures WebSocket URL category support (public/market/private)

Binance is retiring legacy USD-M futures WebSocket URLs on 2026-04-23.
This migrates all fstream methods to use the new categorized URL structure.

* fix: Limit listenKey query-param format to USD-M futures only, not COIN-M

* fix: correct testnet URL in integration tests (fstream not stream)