Before this change: `thought` flags was incorrectly removed if the current agent enables BuiltInPlanner.
After this change:
- When it's BuiltInPlanner, keep the thought flag in content history, so that model has full context of its previous thinking.
- When it's PlanReactPlanner, removes the `thought` flag in content history, so that model sees as-is when the content was generated.
PiperOrigin-RevId: 802737130