Insert newlines after streaming tool results

[jdormit]

This is a fix for #769. It ensures that we insert a gptel-response-separator after a tool result comes in in streaming mode. This prevents weird formatting when gptel-include-tool-results is nil and the tool result came in between two chat responses (see the linked issue for details).

[karthink]

Thanks for the PR.

This is trickier than it appears. I need to test some edge cases when I can before merging.

[jdormit]

Think I've run into one of those edge cases you were talking about @karthink: we should really be inserting the separator before a chat response, rather than after a tool response - otherwise multiple tool calls in a row leads to a bunch of newlines. Not sure if I'll have time to tackle that today but should be able to soon-ish.