feat(core): add retry for transient HTTP and stream-level errors

Add retry mechanism driven by pi settings.json retry.provider config
(timeoutMs, maxRetries, maxRetryDelayMs).

HTTP-level retries handle 429/5xx with exponential backoff and jitter,
respecting Retry-After headers (seconds and HTTP-date formats).

Stream-level retries handle cases where the API returns 200 OK but
sends an error event in the stream body. Retries only when no content
has been emitted yet.

Per-attempt timeout via AbortController with automatic retry. Clean
abort propagation through the retry loop.