fix(common): use enum values in WebSocket stream names

[Amadeus-22]

The generated stream methods are typed to take enums (spot kline(interval: KlineIntervalEnum), ticker_rolling_window(window_size: ...), futures continuous_contract_kline_candlestick_streams(contract_type: ..., interval: ...) and others), but ws_streams_placeholder fills placeholders with str(value), so passing the enum builds a stream name that doesn't exist:

ws_streams_placeholder("<symbol>@kline_<interval>", {"symbol": "BTCUSDT", "interval": KlineIntervalEnum.INTERVAL_1m})
# 'btcusdt@kline_KlineIntervalEnum.INTERVAL_1m'   expected 'btcusdt@kline_1m'

ws_streams_placeholder("<symbol>@ticker_<windowSize>", {"symbol": "BTCUSDT", "window_size": WindowSizeEnum.WINDOW_SIZE_1h})
# 'btcusdt@ticker_windowsizeenum.window_size_1h'

The subscription is accepted but never receives data; the examples only work because they pass .value. The REST (make_serializable) and WebSocket API paths already unwrap enums; this does the same for stream names.

Added TestWsStreamsPlaceholder.test_enum_values_are_unwrapped (kline, continuousKline, ticker window size). It fails on master and passes with the fix; pytest tests/unit in common/ passes (404). flake8 is clean; black --check flags only a pre-existing docstring in RedactedRepr, untouched here.

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