[codex] Fix execution report heartbeat globs

[Pigbibi]

Summary

• Include both YYYY-MM and YYYY_MM month segments when building execution report heartbeat GCS globs.
• Add a targeted test for generated heartbeat report globs across month boundaries.

Root Cause

Strategy run persistence sanitizes run periods for GCS keys, so 2026-06 is stored under 2026_06. The heartbeat workflow only searched YYYY-MM directories and missed valid reports.

Validation

• .venv/bin/python -m pytest -q tests/test_execution_report_heartbeat.py
• env GCP_PROJECT_ID=firstradequant FIRSTRADE_GCS_STATE_BUCKET=firstradequant-runtime-state-1088907247379 FIRSTRADE_STATE_PREFIX=firstrade-platform RUNTIME_HEARTBEAT_NAME=FirstradePlatform RUNTIME_HEARTBEAT_LOOKBACK_HOURS=36 RUNTIME_HEARTBEAT_FAIL_WORKFLOW_ON_ALERT=true python scripts/execution_report_heartbeat.py