Document non-retryable broken executor behavior

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

Author: cursoragent

README.md

@@ -144,6 +144,7 @@ Polling callback contracts are also validated:
 - Polling retries skip non-retryable API client errors (HTTP `4xx`, except retryable `408` request-timeout and `429` rate-limit responses).
 - 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.
+- Broken executor errors 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.
 - Async generator reuse runtime errors (e.g. `asynchronous generator is already running`) are treated as non-retryable and surfaced immediately.