DEV: Fix failing chat spec and add unexpected failure indicator (#20299)

This commit fixes the UpdateUserLastRead spec which was checking
for a message ID that did not exist -- this could fail at times
since message ID 2 could exist. Better to create + destroy a message
since then it's guaranteed we have a unique ID.

This also attempts to clarify a step that we expect to fail which
succeeds instead by adding another emoji next to the success tick and
an explanation text.

Also removes some uses of unless in Services::Base, we generally prefer
to use alternatives, since unless can be hard to parse in a lot of
cases.

Co-authored-by: Loïc Guitaut <loic@discourse.org>

plugins/chat/lib/steps_inspector.rb

@@ -30,9 +30,7 @@ module Chat
       end
 
       def emoji
-        return "❌" if failure?
-        return "✅" if success?
-        ""
+        "#{result_emoji}#{unexpected_result_emoji}"
       end
 
       def steps
@@ -48,6 +46,21 @@ module Chat
       def step_result
         result["result.#{type}.#{name}"]
       end
+
+      def result_emoji
+        return "❌" if failure?
+        return "✅" if success?
+        ""
+      end
+
+      def unexpected_result_emoji
+        " ⚠️#{unexpected_result_text}" if step_result.try(:[], "spec.unexpected_result")
+      end
+
+      def unexpected_result_text
+        return "  <= expected to return true but got false instead" if failure?
+        "  <= expected to return false but got true instead"
+      end
     end
 
     # @!visibility private