Document non-retryable iterator exhaustion callback behavior

Co-authored-by: Shri Sukhani <shrisukhani@users.noreply.github.com>

Author: cursoragent

README.md

@@ -146,6 +146,7 @@ Polling callback contracts are also validated:
 - Cancellation exceptions are treated as non-retryable and are surfaced immediately.
 - Callback contract violations and callback execution failures fail fast with explicit callback-specific errors.
 - Reused coroutine callback errors (e.g. `cannot reuse already awaited coroutine`) are treated as non-retryable and surfaced immediately.
+- Iterator exhaustion callback errors (`StopIteration` / `StopAsyncIteration`) are treated as non-retryable and surfaced immediately.
 
 Example: