[BREAKING] Python: Move single-config fluent methods to constructor parameters (#3693)

* Move single-config fluent methods to constructor parameters

* Updates

* Adjust magentic and group chat

python/samples/getting_started/workflows/parallelism/aggregate_results_of_different_types.py

@@ -73,12 +73,11 @@ class Aggregator(Executor):
 async def main() -> None:
     # 1) Build a simple fan out and fan in workflow
     workflow = (
-        WorkflowBuilder()
+        WorkflowBuilder(start_executor="dispatcher")
         .register_executor(lambda: Dispatcher(id="dispatcher"), name="dispatcher")
         .register_executor(lambda: Average(id="average"), name="average")
         .register_executor(lambda: Sum(id="summation"), name="summation")
         .register_executor(lambda: Aggregator(id="aggregator"), name="aggregator")
-        .set_start_executor("dispatcher")
         .add_fan_out_edges("dispatcher", ["average", "summation"])
         .add_fan_in_edges(["average", "summation"], "aggregator")
         .build()

python/samples/getting_started/workflows/parallelism/fan_out_fan_in_edges.py

@@ -123,13 +123,12 @@ def create_legal_agent() -> ChatAgent:
 async def main() -> None:
     # 1) Build a simple fan out and fan in workflow
     workflow = (
-        WorkflowBuilder()
+        WorkflowBuilder(start_executor="dispatcher")
         .register_agent(create_researcher_agent, name="researcher")
         .register_agent(create_marketer_agent, name="marketer")
         .register_agent(create_legal_agent, name="legal")
         .register_executor(lambda: DispatchToExperts(id="dispatcher"), name="dispatcher")
         .register_executor(lambda: AggregateInsights(id="aggregator"), name="aggregator")
-        .set_start_executor("dispatcher")
         .add_fan_out_edges("dispatcher", ["researcher", "marketer", "legal"])  # Parallel branches
         .add_fan_in_edges(["researcher", "marketer", "legal"], "aggregator")  # Join at the aggregator
         .build()

python/samples/getting_started/workflows/parallelism/map_reduce_and_visualization.py

@@ -261,7 +261,7 @@ async def main():
 
     # Step 1: Create the workflow builder and register executors.
     workflow_builder = (
-        WorkflowBuilder()
+        WorkflowBuilder(start_executor="split_data_executor")
         .register_executor(lambda: Map(id="map_executor_0"), name="map_executor_0")
         .register_executor(lambda: Map(id="map_executor_1"), name="map_executor_1")
         .register_executor(lambda: Map(id="map_executor_2"), name="map_executor_2")
@@ -286,7 +286,6 @@ async def main():
     # Step 2: Build the workflow graph using fan out and fan in edges.
     workflow = (
         workflow_builder
-        .set_start_executor("split_data_executor")
         .add_fan_out_edges(
             "split_data_executor",
             ["map_executor_0", "map_executor_1", "map_executor_2"],