[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/devui/fanout_workflow/workflow.py

@@ -662,8 +662,8 @@ def create_complex_workflow():
         WorkflowBuilder(
             name="Data Processing Pipeline",
             description="Complex workflow with parallel validation, transformation, and quality assurance stages",
+            start_executor=data_ingestion,
         )
-        .set_start_executor(data_ingestion)
         # Fan-out to validation stage
         .add_fan_out_edges(data_ingestion, [schema_validator, quality_validator, security_validator])
         # Fan-in from validation to aggregator

python/samples/getting_started/devui/in_memory_mode.py

@@ -102,8 +102,8 @@ def main():
         WorkflowBuilder(
             name="Text Transformer",
             description="Simple 2-step workflow that converts text to uppercase and adds exclamation",
+            start_executor=upper_executor,
         )
-        .set_start_executor(upper_executor)
         .add_edge(upper_executor, exclaim_executor)
         .build()
     )

python/samples/getting_started/devui/spam_workflow/workflow.py

@@ -392,13 +392,13 @@ legitimate_message_handler = LegitimateMessageHandler(id="legitimate_message_han
 final_processor = FinalProcessor(id="final_processor")
 
 # Build the comprehensive 4-step workflow with branching logic and HIL support
-# Note: No .with_checkpointing() call - DevUI will pass checkpoint_storage at runtime
+# Note: No checkpoint_storage in constructor - DevUI will pass checkpoint_storage at runtime
 workflow = (
     WorkflowBuilder(
         name="Email Spam Detector",
         description="4-step email classification workflow with human-in-the-loop spam approval",
+        start_executor=email_preprocessor,
     )
-    .set_start_executor(email_preprocessor)
     .add_edge(email_preprocessor, spam_detector)
     # HIL handled within spam_detector via @response_handler
     # Continue with branching logic after human approval

python/samples/getting_started/devui/workflow_agents/workflow.py

@@ -132,8 +132,8 @@ workflow = (
     WorkflowBuilder(
         name="Content Review Workflow",
         description="Multi-agent content creation workflow with quality-based routing (Writer → Reviewer → Editor/Publisher)",
+        start_executor=writer,
     )
-    .set_start_executor(writer)
     .add_edge(writer, reviewer)
     # Branch 1: High quality (>= 80) goes directly to publisher
     .add_edge(reviewer, publisher, condition=is_approved)