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/spec/support/chat_service_matcher.rb

@@ -15,6 +15,7 @@ module Chat
       end
 
       def failure_message
+        set_unexpected_result
         message =
           if !step_exists?
             "Expected #{type} '#{name}' (key: '#{step}') was not found in the result object."
@@ -27,6 +28,7 @@ module Chat
       end
 
       def failure_message_when_negated
+        set_unexpected_result
         message = "Expected #{type} '#{name}' (key: '#{step}') to succeed but it failed."
         error_message_with_inspection(message)
       end
@@ -61,6 +63,11 @@ module Chat
         inspector = StepsInspector.new(result)
         "#{message}\n\n#{inspector.inspect}\n\n#{inspector.error}"
       end
+
+      def set_unexpected_result
+        return unless result[step]
+        result[step]["spec.unexpected_result"] = true
+      end
     end
 
     class FailContract < FailStep