subscribe_candle() never returns historical backfill — server sends empty snapshot because CHANNEL_REQUEST always uses contract: "AUTO"

[Section1256]

DXLinkStreamer._channel_request hardcodes "contract": "AUTO" in the CHANNEL_REQUEST message for every event type, including Candle. For Candle subscriptions with a start_time, this means the server never sends historical backfill — it responds with a single FEED_DATA event carrying eventFlags=14 (SNAPSHOT_BEGIN | SNAPSHOT_END | REMOVE_EVENT — dxFeed's convention for an explicit empty snapshot terminator) and all-NaN/zero OHLC fields, even for a highly liquid symbol like AAPL with start_time 90 days back.

Manually driving the dxLink protocol and requesting "contract": "HISTORY" instead of "AUTO" on the same channel, with everything else identical, returns real backfill immediately — dozens of real daily OHLC bars with correct dates, prices, and volume.

Repro (using tastytrade==13.2.3):

from datetime import datetime, timedelta, timezone
from tastytrade import Session, DXLinkStreamer
from tastytrade.dxfeed import Candle

session = Session(...)
async with session:
    await session.refresh(force=True)
    start_time = datetime.now(timezone.utc) - timedelta(days=90)
    async with DXLinkStreamer(session) as streamer:
        await streamer.subscribe_candle(["AAPL"], "1d", start_time=start_time)
        candle = await streamer.get_event(Candle)
        print(candle.time, candle.open, candle.close)
        # -> time equals the requested start_time verbatim (echoed back),
        #    open/close are 0 - not a real bar.

What I found by bypassing the SDK and sending a raw CHANNEL_REQUEST with "parameters": {"contract": "HISTORY"} instead of "AUTO" (same FEED_SETUP/FEED_SUBSCRIPTION messages otherwise): real daily bars for AAPL, SPX, and a front-month futures symbol (/MESU26:XCME) all came back correctly, with realistic dates/prices/volume and proper eventFlags (4 for the live/current-day bar, 0 for closed days).

Reproduced against both the current PyPI release (13.2.3) and unreleased master — _channel_request is unchanged in both.

Suggested fix: either let subscribe_candle()/_channel_request request "HISTORY" for Candle channels specifically, or expose contract as a parameter so callers can choose.

Happy to open a PR if that's useful — wanted to confirm this isn't already-known/by-design first.

[Graeme22]

This is definitely helpful, thanks for opening. One part I don't get is where you say that historical candles are never present--that doesn't line up with my experience at all. I will try to reproduce later.