Fix Poloniex trade parsing

[DeepCogNeural]

Fixes #1113.

What changed

• Parse every entry in Poloniex trades message batches instead of only data[0].
• Set Trade.exchange to self.id and pass the Poloniex trade id via id=str(entry["id"]).
• Use entry["quantity"] as Trade.amount; Poloniex amount is quote notional.
• Add focused unit coverage for the issue sample and a two-trade batch sample.

Validation

• .venv313/bin/python -m pytest tests/unit/test_poloniex.py 'tests/unit/test_exchange.py::test_exchange_playback[POLONIEX]' -q -> 3 passed
• .venv313/bin/python -m flake8 cryptofeed/exchanges/poloniex.py tests/unit/test_poloniex.py -> passed
• git diff --check -> passed

I also attempted .venv313/bin/python -m pytest tests/unit -q; it fails in unrelated non-Poloniex symbol/playback tests with stale fixture/API-shape errors, so I did not treat full unit as a passing signal.

No authenticated, private, or live trading paths were touched.