Refactor AsyncWrite implementation and enhance web crawler error handling

[Unmesh100]

• Updated the poll_flush method in SmolStream to correctly call poll_flush for both Plain and Tls variants.
• Introduced a FetchGuard struct in web-crawler.rs to ensure that an empty string is sent if the fetch operation is dropped before completion, preventing potential deadlocks.
• Improved error handling in the fetch function by ensuring the sender is notified of the fetch completion status.