Document fail-fast behavior for reused coroutine callback errors

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

Author: cursoragent

README.md

@@ -145,6 +145,7 @@ Polling callback contracts are also validated:
 - SDK timeout/polling exceptions (`HyperbrowserTimeoutError`, `HyperbrowserPollingError`) are treated as non-retryable and are surfaced immediately.
 - 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.
 
 Example: