Implement bidirectional handoff system (#1376)

[CodeVoyager007]

Resolves #1376: Bidirectional Handoff System

This PR implements a bidirectional handoff system for orchestrator-style workflows, allowing sub-agents to return control to their parent agent. Key features:

• AgentRegistry and AgentRunner for scalable agent/workflow management
• return_to_parent_handoff and create_bidirectional_handoff_workflow for flexible workflows
• Parallel task execution via run_parallel
• Convenience functions (create_financial_workflow_registry, create_support_workflow_registry)
• Comprehensive test suites in tests/test_bidirectional_handoffs.py and tests/test_registry_handoffs.py
• Documentation in docs/bidirectional_handoffs.md, with updates to docs/index.md, and README.md

Testing

• All tests pass (pytest tests/test_bidirectional_handoffs.py tests/test_registry_handoffs.py -v)
• Covers edge cases (missing agents, circular references, parallel execution)

Checklist

• Code follows SDK conventions
• Tests cover all new functionality
• Documentation updated
• Backward compatible with existing handoffs

Please review and let me know if any changes are needed

[seratch]

Thanks for sending this. We're not planning to add this mechanism in the short term, but sharing your work with communities and knowing what people think would be beneficial for us too.