Raise RatelimitException for HTTP 429 responses

[KetanHegde]

Summary

Closes #478.

This PR raises RatelimitException when a search engine request receives an HTTP 429 response, allowing callers to distinguish rate limiting from other DDGSException errors.

Additionally, _search_sync() now preserves RatelimitException by re-raising it instead of wrapping it as a generic DDGSException.

Changes

• Raise RatelimitException for HTTP 429 responses in BaseSearchEngine.request().
• Preserve RatelimitException in DDGS._search_sync() by re-raising it.

Testing

• Ran ruff check .
• Ran pytest

All tests passed.