DEV: Add policy objects to services

This patch introduces policy objects to chat services. It allows putting
more complex logic in a dedicated class, which will make services
thinner. It also allows providing a reason why the policy failed.

Some change has been made to the service runner too to use more easily
these new policy objects: when matching a failing policy (or any failing
step actually), the result object is now provided to the block. This
way, instead of having to access the reason why the policy failed by
doing `result["result.policy.policy_name"].reason` inside the block,
this one can be simply written like this:
```ruby
  on_failed_policy(:policy_name) { |policy| policy.reason }
```

plugins/chat/lib/service_runner.rb

@@ -12,12 +12,20 @@
 #
 # * +on_success+: will execute the provided block if the service succeeds
 # * +on_failure+: will execute the provided block if the service fails
+# * +on_failed_step(name)+: will execute the provided block if the step named
+#   `name` fails
 # * +on_failed_policy(name)+: will execute the provided block if the policy
 #   named `name` fails
 # * +on_failed_contract(name)+: will execute the provided block if the contract
 #   named `name` fails
-# * +on_model_not_found(name)+: will execute the provided block if the service
-#   fails and its model is not present
+# * +on_model_not_found(name)+: will execute the provided block if the model
+#   named `name` is not present
+# * +on_model_errors(name)+: will execute the provided block if the model named
+#   `name` contains validation errors
+#
+# All the specialized steps receive the failing step result object as an
+# argument to their block. `on_model_errors` receives the actual model so it’s
+# easier to inspect it.
 #
 # Default actions for each of these are defined in [Chat::ApiController#default_actions_for_service]
 #
@@ -28,7 +36,7 @@
 #         flash[:notice] = "Success!"
 #         redirect_to a_path
 #       end
-#       on_failed_policy(:a_named_policy) { redirect_to root_path }
+#       on_failed_policy(:a_named_policy) { |policy| redirect_to root_path, alert: policy.reason }
 #       on_failure { render :new }
 #     end
 #   end
@@ -49,21 +57,42 @@
 #
 
 class ServiceRunner
-  # @!visibility private
-  NULL_RESULT = OpenStruct.new(failure?: false)
   # @!visibility private
   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") 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,
+    on_success: {
+      condition: -> { result.success? },
+      key: [],
+    },
+    on_failure: {
+      condition: -> { result.failure? },
+      key: [],
+    },
+    on_failed_step: {
+      condition: ->(name) { failure_for?("result.step.#{name}") },
+      key: %w[result step],
+    },
+    on_failed_policy: {
+      condition: ->(name = "default") { failure_for?("result.policy.#{name}") },
+      key: %w[result policy],
+      default_name: "default",
+    },
+    on_failed_contract: {
+      condition: ->(name = "default") { failure_for?("result.contract.#{name}") },
+      key: %w[result contract],
+      default_name: "default",
+    },
+    on_model_not_found: {
+      condition: ->(name = "model") { failure_for?("result.model.#{name}") && result[name].blank? },
+      key: %w[result model],
+      default_name: "model",
+    },
+    on_model_errors: {
+      condition: ->(name = "model") do
+        failure_for?("result.model.#{name}") && result["result.model.#{name}"].invalid
+      end,
+      key: [],
+      default_name: "model",
+    },
   }.with_indifferent_access.freeze
 
   # @!visibility private
@@ -104,13 +133,19 @@ class ServiceRunner
   attr_reader :actions
 
   def failure_for?(key)
-    (object.result[key] || NULL_RESULT).failure?
+    object.result[key]&.failure?
   end
 
   def add_action(name, *args, &block)
+    action = AVAILABLE_ACTIONS[name]
     actions[[name, *args].join("_").to_sym] = [
-      -> { instance_exec(*args, &AVAILABLE_ACTIONS[name]) },
-      -> { object.instance_eval(&block) },
+      -> { instance_exec(*args, &action[:condition]) },
+      -> do
+        object.instance_exec(
+          result[[*action[:key], args.first || action[:default_name]].join(".")],
+          &block
+        )
+      end,
     ]
   end