Fix Binance futures L2_BOOK stream name for the default 250ms interval

[MMcollab-dotcom]

Description of code - what bug does this fix / what feature does this add?

BinanceFutures and BinanceDelivery list '250ms' in valid_depth_intervals. With depth_interval='250ms', _stream_names() subscribes to <symbol>@depth@250ms. Binance accepts that subscription but never sends data on it, so L2_BOOK produces nothing and no error is raised.

250ms is Binance's default diff-depth speed on the futures endpoints, and it's served on the bare <symbol>@depth stream. This PR adds a default_depth_interval class attribute: '250ms' on BinanceFutures and BinanceDelivery, None on BinanceBase. _stream_names() leaves out the suffix when the configured interval equals it. Spot is unchanged; @1000ms and the bare stream both work there.

Live check on 2026-09-26, median gap between E of consecutive events:

Endpoint	Stream	Result
fstream /public	btcusdt@depth	252 ms
fstream /public	btcusdt@depth@250ms	no message within 5 s
fstream /public	btcusdt@depth@100ms	102 ms
fstream /public	btcusdt@depth@500ms	502 ms
dstream	btcusd_perp@depth	~265 ms
dstream	btcusd_perp@depth@250ms	no message within 6 s
stream.binance.com	btcusdt@depth@1000ms	1000 ms

With this branch, a FeedHandler with BinanceFutures and BinanceDelivery at depth_interval='250ms' received L2_BOOK deltas every 252 ms and 260 ms (median over 12 s).

• - Tested (live, see above)
• - Changelog updated
• - Tests run and pass (tests/unit: 68 passed; the new test fails on master for the two 250ms cases)
• - Flake8 run and all errors/warnings resolved (ruff check passes on changed files; flake8 shows only existing E501/W391 in untouched lines)
• - Contributors file updated (optional)