Add multi-worker handoff example combining Flows with the worker framework

[aconchillo]

Summary

Adds a new example, examples/multi_worker_handoff.py, demonstrating how Pipecat Flows composes with Pipecat's multi-worker framework (the worker model introduced with Pipecat 1.3.0). It ports the idea from the now-archived pipecat-subagents FlowsAgent example onto the native worker API.

Three workers share a single bus:

• Main worker — owns the transport (STT/TTS) and the single shared LLMContext; bridges user/assistant frames onto the bus via BusBridgeProcessor. It runs no LLM itself.
• Router worker — a plain LLMWorker for open-ended chit-chat, with @tool transfer_to_reservation / end_conversation.
• Reservation worker — a structured Flows conversation (party size → time → availability → confirm), with a transfer_to_router escape hatch at every node.

Only one worker is active at a time; hand-offs (via activate_worker(..., deactivate_self=True)) are seamless. All workers speak into the main worker's shared context_aggregator.

The reservation worker is built by build_reservation_worker(...) returning a plain PipelineWorker (same style as the sensor-controller example) — no subclass. A FlowManager is wired onto the worker and the flow is initialized/resumed from the worker's on_activated event handler.

[markbackman]

LGTM! Thanks for making this update 🙌