#19772: Honor configured heartbeat model overrides in all heartbeat runners

## Summary - add `resolveHeartbeatModelPrimary()` so heartbeat model resolution supports both string and object configs consistently - pass `heartbeatModelOverride` and `suppressToolErrorWarnings` through web heartbeat runs (previously omitted), matching core heartbeat runner behavior - update reply/model selection fallback handling and add regression tests for heartbeat model forwarding and object-form config parsing ## Testing - targeted test execution was not possible in this environment (`pnpm` is unavailable) Fixes #19445