fix: pass params.message directly as initial_message to prevent orphan SUBMITTED rows

When a producer fails before emitting any events, the task is created
via ensure_task_id() using the initial_message passed to TaskManager.
Using params.message directly instead of request_context.message makes
the data flow more explicit and avoids depending on RequestContext's
internal implementation.

Fixes #1067

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Linux2010

src/a2a/server/request_handlers/default_request_handler_v2.py

@@ -231,7 +231,7 @@ async def _setup_active_task(
             context_id=context_id,
             call_context=call_context,
             create_task_if_missing=True,
-            initial_message=request_context.message,
+            initial_message=params.message,
         )
 
         return active_task, request_context