Expose sprint goal in API representer

Adds a nullable `goal` property to `SprintRepresenter`
that resolves the per-project goal text when a
`goal_project:` context is provided. Returns `nil`
otherwise. Updates the OpenAPI schema to match.

Author: myabc

docs/api/apiv3/components/schemas/sprint_model.yml

@@ -23,6 +23,13 @@ properties:
     description: Sprint name
   description:
     $ref: "./formattable.yml"
+  goal:
+    type:
+      - "string"
+      - "null"
+    description: |-
+      The sprint goal for the contextual project. Returns null when no
+      project context is available.
   startDate:
     type:
       - "string"

modules/backlogs/lib/api/v3/sprints/sprint_representer.rb

@@ -60,9 +60,23 @@ class SprintRepresenter < ::API::Decorators::Single
 
         date_property :finish_date
 
+        property :goal,
+                 render_nil: true,
+                 exec_context: :decorator,
+                 getter: ->(*) { goal_text }
+
         date_time_property :created_at
         date_time_property :updated_at
 
+        def initialize(model, current_user:, embed_links: false, goal_project: nil)
+          super(model, current_user:, embed_links:)
+          @goal_project = goal_project
+        end
+
+        def goal_text
+          @goal_project ? represented.goal_for(@goal_project)&.goal : nil
+        end
+
         def _type
           "Sprint"
         end

modules/backlogs/spec/lib/api/v3/sprints/sprint_representer_rendering_spec.rb

@@ -137,6 +137,30 @@
       end
     end
 
+    describe "goal" do
+      context "without project context" do
+        it_behaves_like "property", :goal do
+          let(:value) { nil }
+        end
+      end
+
+      context "with project context and a goal set" do
+        let(:sprint) { create(:sprint, project: workspace) }
+        let(:workspace) { create(:project) }
+        let(:representer) do
+          described_class.new(sprint, current_user:, embed_links:, goal_project: workspace)
+        end
+
+        before do
+          create(:sprint_goal, sprint:, project: workspace, goal: "Deliver MVP")
+        end
+
+        it "renders the goal text" do
+          expect(generated).to be_json_eql("Deliver MVP".to_json).at_path("goal")
+        end
+      end
+    end
+
     describe "createdAt" do
       it_behaves_like "has UTC ISO 8601 date and time" do
         let(:date) { sprint.created_at }