refactor: remove unused Union import from llm_response.py

The Union type is no longer needed since finish_reason is always
a FinishReason enum (never a string after our mapping).

Addresses review comment:
google#3114 (comment)

Author: aperepel

src/google/adk/models/llm_response.py

@@ -16,7 +16,6 @@
 
 from typing import Any
 from typing import Optional
-from typing import Union
 
 from google.genai import types
 from pydantic import alias_generators