Python: Remove workflow factory methods from WorkflowBuilder in favor of a helper instantiation method

[moonbox3]

Summary

Remove the register_executor(factory_func, ...) and register_agent(factory_func, ...) factory methods from WorkflowBuilder (Python). Instead, introduce a helper method that handles instantiation of executors/agents and then builds the Workflow object directly.

Motivation

The current factory-based approach (register_executor(lambda: ..., name=...), register_agent(create_agent, name=...)) requires managing factory callables throughout the codebase, along with async build methods and deferred initialization patterns. This adds complexity for state isolation that can be achieved more simply.

A helper method that creates executor/agent instances upfront and constructs the Workflow object provides:

• Easier state isolation — fresh instances are created per call without needing lambda wrappers or factory registries
• Simpler API — no need to reason about deferred initialization or factory lifecycle
• Less boilerplate — users write a helper function instead of registering factories on a builder

Current API (to be removed)

# packages/core/agent_framework/_workflows/_workflow_builder.py
WorkflowBuilder.register_executor(factory_func: Callable[[], Executor], name: str | list[str]) -> Self
WorkflowBuilder.register_agent(factory_func: Callable[[], SupportsAgentRun], name: str, ...) -> Self

Used in samples like samples/getting_started/workflows/_start-here/step4_using_factories.py.

Proposed approach

Provide a helper method (e.g. on WorkflowBuilder or as a standalone utility) that:

1. Instantiates executors and agents
2. Builds the Workflow object
3. Returns it ready to run

This keeps state isolation simple — each call to the helper creates fresh instances — without requiring factory management in every piece of code.

Scope

• packages/core/agent_framework/_workflows/_workflow_builder.py — remove register_executor and register_agent factory methods
• Update or remove samples/getting_started/workflows/_start-here/step4_using_factories.py
• Update any tests referencing factory registration (e.g. test_workflow_builder.py)
• Update the DeclarativeWorkflowBuilder and WorkflowFactory if they depend on factory methods
• Add the new helper method with documentation and examples