DEV: Move channel creation for category into service (#21167)

This commit moves the category channel creation out
of the Chat::Api::Channel controller and into a
dedicated CreateCategoryChannel service. A follow up
commit will move the DM channel creation out of
the old DirectMessageChannelCreator service.

Also includes a new on_model_errors helper
for chat service class usage, that collects model
validation errors to present in a nice way.

---------

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

plugins/chat/lib/chat/steps_inspector.rb

@@ -66,6 +66,7 @@ module Chat
     # @!visibility private
     class Model < Step
       def error
+        return result[name].errors.inspect if step_result.invalid
         step_result.exception.full_message
       end
     end

plugins/chat/lib/service_runner.rb

@@ -55,9 +55,15 @@ class ServiceRunner
   AVAILABLE_ACTIONS = {
     on_success: -> { result.success? },
     on_failure: -> { result.failure? },
+    on_failed_step: ->(name) { failure_for?("result.step.#{name}") },
     on_failed_policy: ->(name = "default") { failure_for?("result.policy.#{name}") },
     on_failed_contract: ->(name = "default") { failure_for?("result.contract.#{name}") },
-    on_model_not_found: ->(name = "model") { failure_for?("result.model.#{name}") },
+    on_model_not_found: ->(name = "model") do
+      failure_for?("result.model.#{name}") && result[name].blank?
+    end,
+    on_model_errors: ->(name = "model") do
+      failure_for?("result.model.#{name}") && result["result.model.#{name}"].invalid
+    end,
   }.with_indifferent_access.freeze
 
   # @!visibility private