Centralize retry decisions and delays in CodexErr (#46540)

## What changed

Replace `is_retryable()` with `retry_delay(retry_count)`, which returns `None` for terminal errors and a delay for retryable errors. Prefer server advice when present; otherwise use exponential backoff with jitter. Keep `server_retry_delay()` for callers that need only the server-provided delay.

Route sampling and remote compaction errors through the shared response stream handler, which now checks retryability and obtains the delay together. Preserve server-only retry timing when the retry budget is exhausted.

## Testing

Add coverage for attempt-dependent backoff, zero-duration server advice, and terminal errors that remain non-retryable despite server advice. Update API mapping and provider tests to use the combined retry API.

GitOrigin-RevId: 903f92935e71dea043afe1fad8ccb9389471aa75

codex-rs/codex-client/src/retry.rs

@@ -98,6 +98,7 @@ where
                     .should_retry(&err, attempt, policy.max_attempts) =>
             {
                 let retry_attempt = attempt + 1;
+                // TODO(anp): Respect Retry-After from HTTP responses before retrying the request.
                 let delay = backoff(policy.base_delay, retry_attempt);
                 crate::record_retry!(retry_attempt, delay, RetryOperation::HttpRequest);
                 tokio::time::sleep(delay).await;