gh-103462: Ensure SelectorSocketTransport.writelines registers a writer when data is still pending

[alisaifee]

Description

When the payload used with SelectorSocketTransport.writelines is too large to write in one call to either socket.send or socket.sendmsg the remaining buffer needs to be handled by registering a write handler.

Related Pull Request / Issue

• Implement zero copy writes in SelectorSocketTransport in asyncio #91166

• Issue: Calling SelectorSocketTransport.writelines with a very large payload doesn't get completely written to the socket #103462