fix: pass start_executor to WorkflowBuilder constructor

In agent-framework 1.1.1, WorkflowBuilder requires start_executor as a
mandatory keyword argument. Moved from .set_start_executor() chain call
to constructor parameter.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Prachig-Microsoft

src/ContentProcessorWorkflow/src/steps/claim_processor.py

@@ -151,7 +151,7 @@ def _init_workflow(self) -> Workflow:
         """
 
         workflow = (
-            WorkflowBuilder()
+            WorkflowBuilder(start_executor="document_processing")
             .register_executor(
                 lambda: DocumentProcessExecutor(
                     id="document_processing", app_context=self.app_context
@@ -172,7 +172,6 @@ def _init_workflow(self) -> Workflow:
                 lambda: GapExecutor(id="gap_analysis", app_context=self.app_context),
                 name="gap_analysis",
             )
-            .set_start_executor("document_processing")
             # Edges define the execution flow and can include conditions for branching logic.
             # In this case, we conditionally branch to the RAI analysis step based on the
             # application configuration, allowing it to be toggled on/off without code changes.