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/spec/lib/service_runner_spec.rb

@@ -210,8 +210,22 @@ RSpec.describe ServiceRunner do
       context "when the service policy fails" do
         let(:service) { FailedPolicyService }
 
-        it "runs the provided block" do
-          expect(runner).to eq :policy_failure
+        context "when not using the block argument" do
+          it "runs the provided block" do
+            expect(runner).to eq :policy_failure
+          end
+        end
+
+        context "when using the block argument" do
+          let(:actions) { <<-BLOCK }
+              proc do
+                on_failed_policy(:test) { |policy| policy == result["result.policy.test"] }
+              end
+            BLOCK
+
+          it "runs the provided block" do
+            expect(runner).to be true
+          end
         end
       end
 
@@ -234,8 +248,22 @@ RSpec.describe ServiceRunner do
       context "when the service contract fails" do
         let(:service) { FailedContractService }
 
-        it "runs the provided block" do
-          expect(runner).to eq :contract_failure
+        context "when not using the block argument" do
+          it "runs the provided block" do
+            expect(runner).to eq :contract_failure
+          end
+        end
+
+        context "when using the block argument" do
+          let(:actions) { <<-BLOCK }
+              proc do
+                on_failed_contract { |contract| contract == result["result.contract.default"] }
+              end
+            BLOCK
+
+          it "runs the provided block" do
+            expect(runner).to be true
+          end
         end
       end
 
@@ -250,7 +278,7 @@ RSpec.describe ServiceRunner do
 
     context "when using the on_model_not_found action" do
       let(:actions) { <<-BLOCK }
-          ->(*) do
+          proc do
             on_model_not_found(:fake_model) { :no_model }
           end
         BLOCK
@@ -259,8 +287,22 @@ RSpec.describe ServiceRunner do
         context "when the service fails without a model" do
           let(:service) { FailureWithModelService }
 
-          it "runs the provided block" do
-            expect(runner).to eq :no_model
+          context "when not using the block argument" do
+            it "runs the provided block" do
+              expect(runner).to eq :no_model
+            end
+          end
+
+          context "when using the block argument" do
+            let(:actions) { <<-BLOCK }
+                proc do
+                  on_model_not_found(:fake_model) { |model| model == result["result.model.fake_model"] }
+                end
+              BLOCK
+
+            it "runs the provided block" do
+              expect(runner).to be true
+            end
           end
         end
 
@@ -294,7 +336,7 @@ RSpec.describe ServiceRunner do
 
     context "when using the on_model_errors action" do
       let(:actions) { <<-BLOCK }
-          ->(*) do
+          proc do
             on_model_errors(:fake_model) { :model_errors }
           end
         BLOCK
@@ -302,8 +344,22 @@ RSpec.describe ServiceRunner do
       context "when the service fails with a model containing errors" do
         let(:service) { FailureWithModelErrorsService }
 
-        it "runs the provided block" do
-          expect(runner).to eq :model_errors
+        context "when not using the block argument" do
+          it "runs the provided block" do
+            expect(runner).to eq :model_errors
+          end
+        end
+
+        context "when using the block argument" do
+          let(:actions) { <<-BLOCK }
+              proc do
+                on_model_errors(:fake_model) { |model| model == OpenStruct.new(invalid?: true) }
+              end
+            BLOCK
+
+          it "runs the provided block" do
+            expect(runner).to be true
+          end
         end
       end