feat(providers): retry upstream HTTP 503 like 429

Extend execute_with_retry with retryable_upstream_status for 429/503
shared backoff, reactive block, and TRACE provider.retry.scheduled.
Map InternalServerError(503) to APIError with preserved status.
Native transport closes-and-raises on 503 like 429 for clean retries.
Add NIM/OpenAI-compat and rate limiter tests.