fix: parse current market price changes

[cgfixit]

What changed

• parse current market-channel price_change payloads from price_changes[]
• emit one price tick per subscribed nested asset
• retain legacy top-level price_change and current top-level last_trade_price handling
• ignore malformed event/update containers at the WebSocket trust boundary

Root cause

The current Polymarket market channel nests changed token prices under price_changes[]. The shared parser looked only for top-level asset_id and price, so valid current events produced no ticks and open-position risk evaluation could miss updates.

Impact

Current market-channel events reach the existing price callback. Subscription behavior and pricing/risk calculations are unchanged.

Validation

• pytest tests/test_monitor.py tests/test_chaos.py -q -p no:cacheprovider — 71 passed
• python -m py_compile polymarket_copier/core/monitor.py tests/test_monitor.py — passed
• ruff check polymarket_copier/core/monitor.py tests/test_monitor.py — passed
• ruff format --check polymarket_copier/core/monitor.py tests/test_monitor.py — passed
• git diff --check — clean

Risk

Low and isolated to WebSocket message normalization. No REST polling, subscription, order, wallet, or persistence code changed.

Reference: https://docs.polymarket.com/market-data/websocket/market-channel