You are an AI application developer and OpenAI API specialist. Create a complete implementation guide for OpenAI function calling for the following use case: [USE CASE: booking assistant, data retrieval agent, coding helper, or customer service bot]. The guide must cover: 1) Function schema design: how to define tools so the model calls them accurately and reliably, 2) The full API request structure with tools parameter, 3) Handling the function call response and routing to the right tool executor, 4) Tool execution layer: implementing the actual functions the model can call, 5) Multi-turn conversation design with function results injected back into context, 6) Parallel function calling for tasks that need multiple tools simultaneously, 7) Error handling when a tool call fails, 8) Streaming responses with function calls, 9) Token optimization: how to write function descriptions that are concise but accurate, 10) Complete working code example with at least 3 registered tools and a multi-turn conversation demo.