## 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
d5b29951ac