fix(api): require heartbeat SigNoz rate-limit checks

[samuelclay]

Summary

• require heartbeat investigations to run a realtime-replica SigNoz checklist before final answers
• include targeted searches for Too Many Requests, 429, ConnectError, HTTP error, TransportStep boot failures, and livekit_ffi
• add a unit test that locks the checklist into the generated Watch Agent prompt

Validation

• uv run ruff format api/workflows/heartbeat_investigation.py tests/test_heartbeat_investigation_workflow.py
• uv run ruff check api/workflows/heartbeat_investigation.py tests/test_heartbeat_investigation_workflow.py
• uv run pytest tests/test_heartbeat_investigation_workflow.py
• just build-one api
• docker run --rm -i --entrypoint /app/.venv/bin/python centaur-api:latest - prompt checklist assertion

Local deploy smoke

Blocked on this workstation: there is no local Kubernetes context and the required local secret env vars are absent (OP_SERVICE_ACCOUNT_TOKEN, OP_VAULT, SLACK_BOT_TOKEN, SLACK_SIGNING_SECRET, SLACKBOT_API_KEY).

Fork tracking

Pending follow-up commit after PR URL is known.