This PR adds automatic handling of RATE_LIMIT_HIT errors in the Zulip API client. When a rate limit error is encountered, the client will now automatically wait for the specified period (from the Retry-After header if available) and retry the request, rather than requiring each application to implement this logic separately.

Fixes: The issue where applications need to implement their own rate limit handling logic when using the Zulip API client.

Changes include:

1. Enhanceddo_api_query method to detect RATE_LIMIT_HIT errors
2. Added logic to extract and use the Retry-After header value
3. Implemented automatic retry mechanism with appropriate sleep duration
4. Added fallback to existing error retry mechanism when no Retry-After header is provided
5. Added comprehensive tests for the new functionality
6. Updated README.md with documentation about the automatic rate limit handling

Code example of the implementation:

# Handle rate limiting automaticallyifjson_result.get("result")=="error"andjson_result.get("code")=="RATE_LIMIT_HIT":retry_after=None# Check for Retry-After header (in seconds)if"Retry-After"inres.headers:try:retry_after=int(res.headers["Retry-After"])except (ValueError,TypeError):pass# If we have a valid retry_after value, sleep and retryifretry_afterandretry_after>0:ifself.verbose:print(f"Rate limit hit. Retrying after{retry_after} seconds...")time.sleep(retry_after)continue# If no valid retry_after header, use a default backoffeliferror_retry(" (rate limited)"):continue