refactor: remove unused variable and simplify message processing in _agent_graph.py

- Removed the unused `parts` variable in `UserPromptNode`.
- Simplified message processing logic in `_process_message_history` by directly using the processor without casting for async processing.

Author: tradeqvest

pydantic_ai_slim/pydantic_ai/_agent_graph.py

@@ -229,7 +229,6 @@ async def run(  # noqa: C901
         # Build the run context after `ctx.deps.prompt` has been updated
         run_context = build_run_context(ctx)
 
-        parts: list[_messages.ModelRequestPart] = []
         if messages:
             await self._reevaluate_dynamic_prompts(messages, run_context)
 
@@ -1152,8 +1151,7 @@ async def _process_message_history(
             if takes_ctx:
                 messages = await processor(run_context, messages)
             else:
-                async_processor = cast(_HistoryProcessorAsync, processor)
-                messages = await async_processor(messages)
+                messages = await processor(messages)
         else:
             if takes_ctx:
                 sync_processor_with_ctx = cast(_HistoryProcessorSyncWithCtx[DepsT], processor)