fix: Add buffer to the write file option

PiperOrigin-RevId: 785546915

Author: Jianfeng Zeng

src/google/adk/cli/fast_api.py

@@ -856,7 +856,7 @@ async def builder_build(files: list[UploadFile]) -> bool:
         os.makedirs(agent_dir, exist_ok=True)
         file_path = os.path.join(agent_dir, filename)
 
-        with open(file_path, "w") as buffer:
+        with open(file_path, "wb") as buffer:
           shutil.copyfileobj(file.file, buffer)
 
       except Exception as e: