refactor: remove unused variable

- Removed the unused `parts` variable in `UserPromptNode`.

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)