Python: Simplify agent executor registration with dict-based API

[holtvogt]

Summary

Consider using dictionary-based registration for register_agent() calls to reduce verbosity:

agent-framework/python/samples/getting_started/workflows/agents/azure_chat_agents_tool_calls_with_feedback.py

Lines 247 to 248 in ef79862

	.register_agent(create_writer_agent, name="writer_agent")
	.register_agent(create_final_editor_agent, name="final_editor_agent")

Context

# Remove the need to make these repetitive register_executor calls
.register_agent(lambda: CustomAgent(name="custom"), name="custom")

Action Items

• Design simpler agent registration via dictionaries

Reference

Similar to #3551