Function Call Dispatcher

#Learning Objectives

• Wrap a tool handler in a per-call timeout that returns a typed error instead of hanging the loop.
• Apply exponential backoff retry with jitter and a maximum attempt count.
• Deduplicate retries on an idempotency key so a retry that races with a slow original does not run twice.
• Map handler exceptions and transport faults onto a single error envelope the harness loop already understands.
• Bound parallel dispatch with a concurrency limit so a fan-out of forty tool calls does not exhaust the event loop.