feat: add ingest retries for 429 and 503 with maxRetries

[rmorehig]

Summary

• add optional ingest retry config in IngestOptions via top-level maxRetries
• keep HTTP 429 retries header-driven (Retry-After / X-RateLimit-Reset)
• add HTTP 503 retries using SDK default exponential backoff (internal defaults)
• apply the same maxRetries cap to both 429 and 503 retries
• drain retryable response bodies before retrying
• simplify API by removing nested retry object from ingest options
• update README examples and unit tests for 429/503 behavior

Validation

• pnpm typecheck
• pnpm exec vitest run src/api/api.test.ts