test: consume heartbeat wait coroutines

[cgfixit]

What

• Replace three global asyncio.wait_for test stubs with per-event Event.wait stubs.
• Exercise the production heartbeat's real timeout wrapper.
• Eliminate leaked coroutine warnings from the WebSocket heartbeat tests.

Why

The tests passed while leaving each Event.wait() coroutine unconsumed. On origin/main, the focused run reported 3 passed with 5 RuntimeWarning: coroutine 'Event.wait' was never awaited warnings. Promoting those warnings to errors exposed the test defect.

This is tests-only. Production code, trading behavior, configuration, and dependencies are unchanged.

Validation

• Baseline on origin/main: focused heartbeat run -> 3 passed, 5 RuntimeWarnings
• python -m pytest tests/test_monitor.py::TestTradeMonitor::test_ws_heartbeat_sends_polymarket_ping tests/test_monitor.py::TestTradeMonitor::test_ws_heartbeat_subscription_wake_does_not_ping tests/test_monitor.py::TestMonitorRunAndSubscriptionRefresh::test_ws_heartbeat_pushes_subscription_update -q -W error::RuntimeWarning -> 3 passed
• python -m pytest tests/test_monitor.py -q -W error::RuntimeWarning -> 54 passed
• python -m pytest -v -m "not integration" -> 624 passed
• powershell -File scripts/check-lint.ps1 -> Ruff lint and format passed
• python -m mypy polymarket_copier --ignore-missing-imports --no-strict-optional -> success
• git diff --check -> clean

Risk To Monitor

Low. The change only narrows test mocking from the global scheduler helper to the heartbeat's event wait method. CI timing and production runtime behavior are unaffected.