Document retryable 408 and 429 polling client errors

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

Author: cursoragent

README.md

@@ -141,7 +141,7 @@ Polling callback contracts are also validated:
 
 - Sync polling helpers require non-awaitable callback return values.
 - Async polling helpers require awaitable status/page/retry callbacks.
-- Polling retries skip non-retryable API client errors (HTTP `4xx`, except `429` rate-limit responses).
+- 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.
 - Callback contract violations and callback execution failures fail fast with explicit callback-specific errors.