REFACTOR: Simplify tool invocation by removing bot_user and llm parameters (#603)

* Well, it was quite a journey but now tools have "context" which
can be critical for the stuff they generate

This entire change was so Dall E and Artist generate images in the correct context

* FIX: improve error handling around image generation

- also corrects image markdown and clarifies code

* fix spec

lib/ai_bot/tools/github_pull_request_diff.rb

@@ -43,7 +43,7 @@ module DiscourseAi
           @url
         end
 
-        def invoke(_bot_user, llm)
+        def invoke
           api_url = "https://api.github.com/repos/#{repo}/pulls/#{pull_id}"
           @url = "https://github.com/#{repo}/pull/#{pull_id}"