Implement rate-limit handling

[codebestia]

Description

When the API returns HTTP 429, the SDK should parse the Retry-After header (seconds) and either automatically wait and retry (if within max_retries) or raise RateLimitError with the wait time attached. The caller should never need to implement their own 429 backoff.

Proposed Steps

• In the response parser, detect HTTP 429 before raising a generic error.
• Parse Retry-After header (integer seconds) and attach it to the RateLimitError.
• If auto-retry is enabled and max_retries is not exhausted, sleep for retry_after seconds then retry.
• In the async client, use asyncio.sleep instead of time.sleep.

Acceptance Criteria

• A 429 response raises RateLimitError with a retry_after attribute.
• When retrying, the client waits at least Retry-After seconds before the next attempt.
• If Retry-After is absent, the client falls back to exponential backoff.
• Async client uses non-blocking sleep.

[bukkybyte]

Allow me work on this and dwliver PR within 2 days