Fix Coinbase public market data auth

[DeepCogNeural]

Summary

• load Coinbase symbols from the public /market/products endpoint so public symbol lookup no longer requires Coinbase API keys
• avoid adding Coinbase websocket signing fields when no API keys are configured, while preserving the signed branch when keys are present
• subscribe to and ignore the documented public heartbeats channel

Fixes #1114.

Tests

• .venv313/bin/python -m pytest tests/unit/test_coinbase.py -q (4 passed)
• git diff --check

Manual checks

• reproduced the pre-fix no-key Coinbase.symbols(config={}, refresh=True) failure as a credential ValueError
• reproduced the pre-fix no-key subscribe() path entering Coinbase signing and failing on missing key_secret
• verified with a mocked public products response that Coinbase.symbols(config={}, refresh=True) reads /api/v3/brokerage/market/products with headers=None
• verified with a recording connection that no-key public subscriptions send unsigned market_trades and heartbeats payloads
• verified with a keyed config that subscription payloads still include api_key, timestamp, and signature