chore: include contributing/ folder in autoformat.sh

Also formatted the files in that folder in this same commit.

PiperOrigin-RevId: 766885306

contributing/samples/hello_world/agent.py

@@ -65,6 +65,7 @@ async def check_prime(nums: list[int]) -> str:
       else f"{', '.join(str(num) for num in primes)} are prime numbers."
   )
 
+
 root_agent = Agent(
     model='gemini-2.0-flash',
     name='data_processing_agent',